The microplate version of the leukocyte adherence inhibition (LAI) test was evaluated in murine and human studies. It was used in parallel with the microcytotoxicity assay, lymphotoxin assay, leukocyte migration inhibition, and lymphocyte stimulation tests in a transplantable murine tumor model, and it compared favorably with these established techniques for the detection of cellular immunity. The LAI test detected both primary and secondary anamnestic responses in Bacillus Calmette-Guérin-primed mice, and it displayed sensitivity to host humoral factors comparable to that seen with other tests. The LAI phenomenon was shown to be mediated by a soluble supernatant factor liberated by antigen-exposed immune leukocytes in the mouse and by concanavalin A-stimulated human leukocytes. In the mouse, deliberate depletion of T-cells ablates LAI reactivity in cells taken at the peak of a primary response; however, immune serum "arms" non-thymus-dependent cells taken from unimmunized hosts. In the mouse, the results of LAI tests correlate with other established techniques, when purified protein derivative reactivity was assessed in spleen cells from Bacillus Calmette-Guérin-immunized mice. Comparable correlation is not found when reactivity to this antigen is assessed in human peripheral blood samples from unimmunized donors.