A method of assaying for plasma homocysteine in a whole blood sample taken from a human or vascularized non-human animal subject, said method comprising: a) contacting a whole blood sample from said subject with the following reagents - a liquid diluent, a reducing agent, a homocysteine-converting enzyme, an inhibitor of the homocysteine converting reaction of the homocysteine-converting enzyme, a cell-lysing agent, a color-labelled binding agent capable of binding to a conversion product of said homocysteine-converting enzyme, and a particulate capable of competing with said conversion product for binding to said binding agent; b) following contact with said reagents, drawing said sample through a membrane having a porosity sufficient to allow passage therethrough of said binding agent in its unbound state and when bound to said conversion product but insufficient to allow passage therethrough of said binding agent when bound to said particulate; c) detecting the color-label of said binding agent retained on said membrane; d) determining therefrom an indication of the plasma homocysteine concentration by applying a correction factor dependent on red blood cell concentration in said whole blood sample; and e) optionally but preferably presenting the plasma homocysteine concentration as a visible or electronic signal; wherein contact with said reagents is sequential or simultaneous subject to the provisos that: i) contact with said lysing agent occurs after contact with said diluent, enzyme and reducing agent; ii) contact with said binding agent and said particulate does not involve contact with a liquid containing both said binding agent and said particulate; and iii) contact with said inhibitor occurs after contact with the diluent, enzyme and reducing agent and no later than contact with the lysing agent. a) contacting a whole blood sample from said subject with the following reagents - a liquid diluent, a reducing agent, a homocysteine-converting enzyme, an inhibitor of the homocysteine converting reaction of the homocysteine-converting enzyme, a cell-lysing agent, a color-labelled binding agent capable of binding to a conversion product of said homocysteine-converting enzyme, and a particulate capable of competing with said conversion product for binding to said binding agent; b) following contact with said reagents, drawing said sample through a membrane having a porosity sufficient to allow passage therethrough of said binding agent in its unbound state and when bound to said conversion product but insufficient to allow passage therethrough of said binding agent when bound to said particulate; c) detecting the color-label of said binding agent retained on said membrane; d) determining therefrom an indication of the plasma homocysteine concentration by applying a correction factor dependent on red blood cell concentration in said whole blood sample; and e) optionally but preferably presenting the plasma homocysteine concentration as a visible or electronic signal; i) contact with said lysing agent occurs after contact with said diluent, enzyme and reducing agent; ii) contact with said binding agent and said particulate does not involve contact with a liquid containing both said binding agent and said particulate; and iii) contact with said inhibitor occurs after contact with the diluent, enzyme and reducing agent and no later than contact with the lysing agent.