Groups of mice were neonatally thymectomized and treated with antithymocyte serum (ATS) prior to challenge infection with viable yeast phase (YP) Histoplasma capsulatum G-17M. Moderate leucocytosis and moderate lymphopenia were seen in immunodeficient animals after infection. Surviving immunodeficient mice exhibited low levels of migration inhibition activity, while peritoneal exudate cells and spleen cells harvested from surviving infected and untreated normal mice showed significant migration inhibition in the presence of histoplasmin antigen. The LD50 values for YP cells of H. capsulatum were 1.1 x 10(6) for normal untreated mice, 6.0 x 10(5) for thymectomized mice, and 6.3 x 10(5) for ATS-treated mice. Thymectomized mice that also received ATS treatment exhibited an LD50 of 1.7 x 10(5) and were 6.5 times more susceptible to infection then normal mice. Mice which were either thymectomized or treated with ATS were 1.7 times as susceptible as normal mice to infection with H. capsulatum. The criterion of susceptibility is a decrease in the LD50 value.