Resident peritoneal macrophages, spleen macrophages and Kupffer cells isolated from normal CBA mice were treated with supernatants from spleen cells of normal or immunized mice, and cultured in the presence of heat-killed Listeria monocytogenes. The capacity of the macrophages, infected in vitro, to control the growth of Listeria was tested. In macrophages treated with supernatant from normal spleen cells, the organisms multiplied extensively during 24 h but in those treated with supernatant from immune spleen cells, growth was greatly inhibited. Macrophages isolated from mice irradiated with 8-9.5 Gy and treated with immune spleen cell supernatant, were as efficient or even more than those from unirradiated mice. The use of multispot slides proved to be a convenient and economical means of culturing and examining cells.