A container (1; 10; 100) for in vitro diagnostic comprising: a lower receptacle designed to contain, before using the container (1; 10; 100), a non-toxic liquid, having a side wall provided with an external thread and ending with a circular mouth, an upper receptacle designed to contain, before using the container (1; 10; 100), a toxic liquid, having a side wall provided with an external thread and ending with a circular mouth smaller in diameter than the circular mouth of the lower receptacle, and threaded coupling means longitudinally engaging said external threads (21, 31) for the butt joint of the lower receptacle with the upper receptacle, and having a transversal septum against which the lower receptacle and the upper receptacle abut when the container (1; 10; 100) is completely closed, the threaded coupling means consisting of a single connecting sleeve, comprising - a lower portion having a side wall with an internal thread engaging the external thread of the lower receptacle, - an upper portion having a side wall of a given length with an internal thread engaging the external thread of the upper receptacle, the internal thread of the upper portion of the connecting sleeve having a retaining projection for limiting the removal stroke of the circular mouth of the upper receptacle from the transversal septum of the connecting sleeve by partial unscrewing of the upper receptacle; the transversal septum separating the lower portion of the connecting sleeve from its upper portion 42) and being provided with a plurality of transfer openings; characterized in that the transversal septum is provided with at least one vent opening; and in that the upper receptacle has on its side wall an abutting step-shaped circumferential projection for closing an interspace formed between the side wall of the upper portion of the connecting sleeve and the side wall of the upper receptacle, the distance of the horizontal plane of the step of the circumferential projection from the circular mouth of the upper receptacle being equal to said length of the upper portion of the connecting sleeve, in order to simultaneously close the lower receptacle and the upper receptacle when both are completely screwed into the respective lower portion and upper portion of the connecting sleeve. a lower receptacle designed to contain, before using the container (1; 10; 100), a non-toxic liquid, having a side wall provided with an external thread and ending with a circular mouth, an upper receptacle designed to contain, before using the container (1; 10; 100), a toxic liquid, having a side wall provided with an external thread and ending with a circular mouth smaller in diameter than the circular mouth of the lower receptacle, and threaded coupling means longitudinally engaging said external threads (21, 31) for the butt joint of the lower receptacle with the upper receptacle, and having a transversal septum against which the lower receptacle and the upper receptacle abut when the container (1; 10; 100) is completely closed, the threaded coupling means consisting of a single connecting sleeve, comprising - a lower portion having a side wall with an internal thread engaging the external thread of the lower receptacle, - an upper portion having a side wall of a given length with an internal thread engaging the external thread of the upper receptacle, the internal thread of the upper portion of the connecting sleeve having a retaining projection for limiting the removal stroke of the circular mouth of the upper receptacle from the transversal septum of the connecting sleeve by partial unscrewing of the upper receptacle; - a lower portion having a side wall with an internal thread engaging the external thread of the lower receptacle, - an upper portion having a side wall of a given length with an internal thread engaging the external thread of the upper receptacle, the internal thread of the upper portion of the connecting sleeve having a retaining projection for limiting the removal stroke of the circular mouth of the upper receptacle from the transversal septum of the connecting sleeve by partial unscrewing of the upper receptacle; the transversal septum separating the lower portion of the connecting sleeve from its upper portion 42) and being provided with a plurality of transfer openings;