Growth of Mycobacterium kansasii in intravenously infected mice ceases when the spleen cells express an enhanced non-specific resistance to a secondary challenge. Mice inoculated with 10(6) CFU M. kanasii 1203 develop a population of splenic T cells which are able to transfer protection passively to sublethally-irradiated syngeneic recipients when challenged with M. kansasii. Although the T-cell activated macrophages were unable to eliminate the mycobacteria from the spleen, they were able to prevent further growth of the organism in vivo. When mice which lack T cells (congenitally athymic, or 'nude' mice) were infected with M. kansasii, the cellular defences were unable to halt the progressive growth of the challenge organisms within the tissues. When normal mice were inoculated with large numbers of viable M. kansasii 1203 (up to 5 X 10(7) CFU), the activated macrophages within the spleen were capable of limiting the further growth of the bacterial population in vivo, but with no T-cell response capable of adoptively immunizing naive recipients against a secondary M. kansasii challenge. Thus, it seems likely that M. kansasii can induce the formation of activated macrophages by two separate mechanisms: one is a T-cell dependent process which occurs in mice inoculated with moderate doses (10(6) CFU) of M. kansasii, while the other is T-cell independent and occurs when a large infectious inoculum is employed.