A medical injector comprising: a body having a proximal end and a distal end; a reservoir disposed in said body, said reservoir including at least first and second mixable components (22, 24), at least one stopper (26, 28) being associated with said reservoir, distal advancement of said stopper (26, 28) a predetermined distance causing mixing of said mixable components (22, 24); a plunger disposed in said body; a knob stem disposed in said body; a knob fixed to said knob stem so as to be rotatable therewith; a sleeve telescopingly disposed over said knob stem and configured to engage said plunger, wherein said knob stem and said sleeve having cooperating elements formed thereon which selectively permit rotation of said knob stem to be transmitted to said sleeve so that said knob stem and said sleeve may rotate together; a spring for urging said sleeve distally; and, a releaseable retainer for releasably retaining said sleeve in a first position against force of said spring, said releaseable retainer including: a detent formed on one of said body and said sleeve; and, a channel characterized in that the channel formed in the other of said body and said sleeve, said channel having a first part disposed partially circumferentially about a longitudinal axis of said sleeve, and a second part extending from said first part and disposed generally parallel to said longitudinal axis, said channel being formed to receive said detent and to permit sliding movement of said detent therealong; wherein, said sleeve is retained in said first position with said detent being located in said first part of said channel, and, wherein, rotation of said knob causes said detent to move into said second part of said channel thus allowing said spring to displace said sleeve distally, distal movement of said sleeve causing distal movement of said plunger which causes distal movement of said stopper (26, 28) thus causing mixing of said mixable components (22, 24). a body having a proximal end and a distal end; a reservoir disposed in said body, said reservoir including at least first and second mixable components (22, 24), at least one stopper (26, 28) being associated with said reservoir, distal advancement of said stopper (26, 28) a predetermined distance causing mixing of said mixable components (22, 24); a plunger disposed in said body; a knob stem disposed in said body; a knob fixed to said knob stem so as to be rotatable therewith; a sleeve telescopingly disposed over said knob stem and configured to engage said plunger, wherein said knob stem and said sleeve having cooperating elements formed thereon which selectively permit rotation of said knob stem to be transmitted to said sleeve so that said knob stem and said sleeve may rotate together; a spring for urging said sleeve distally; and, a releaseable retainer for releasably retaining said sleeve in a first position against force of said spring, said releaseable retainer including: a detent formed on one of said body and said sleeve; and, a channel characterized in that the channel formed in the other of said body and said sleeve, said channel having a first part disposed partially circumferentially about a longitudinal axis of said sleeve, and a second part extending from said first part and disposed generally parallel to said longitudinal axis, said channel being formed to receive said detent and to permit sliding movement of said detent therealong; wherein, said sleeve is retained in said first position with said detent being located in said first part of said channel, and, wherein, rotation of said knob causes said detent to move into said second part of said channel thus allowing said spring to displace said sleeve distally, distal movement of said sleeve causing distal movement of said plunger which causes distal movement of said stopper (26, 28) thus causing mixing of said mixable components (22, 24). a detent formed on one of said body and said sleeve; and, a channel characterized in that the channel formed in the other of said body and said sleeve, said channel having a first part disposed partially circumferentially about a longitudinal axis of said sleeve, and a second part extending from said first part and disposed generally parallel to said longitudinal axis, said channel being formed to receive said detent and to permit sliding movement of said detent therealong; wherein, said sleeve is retained in said first position with said detent being located in said first part of said channel, and, wherein, rotation of said knob causes said detent to move into said second part of said channel thus allowing said spring to displace said sleeve distally, distal movement of said sleeve causing distal movement of said plunger which causes distal movement of said stopper (26, 28) thus causing mixing of said mixable components (22, 24).