Peritoneal macrophages from both congenitally athymic ("nude") mice and heterozygous littermates were activated by pyran copolymer or by Corynebacterium parvum vaccine. C parvum did not produce an increase in the number of peritoneal macrophages in nude mice, although it did produce a typical splenomegaly. Pyran produced an even greater influx of macrophages in the peritoneum of nude mice, when compared to normal mice, but did not produce splenomegaly in nude mice. Pyran- and C parvum-induced splenomegaly were accompanied by an increase in the apparent T-cell population of germinal centers. These experiments indicate that: 1) Macrophage activation, per se, by either C parvum, is a thymus-independent event; 2) Macrophage mobilization, as determined by organomegaly or PEC number, does not have an obligatory requirement for T-cells (depending on the agent used); 3) Macrophage activation may not always correlate with mobilization; and 4) Mechanisms for attracting and sequestering macrophages in the peritoneum may be different from those of the spleen.