Splenomegaly resulting from Trypanosoma musculi infection was found to be dependent upon a functioning T-lymphocyte system. When both humoral and cell-mediated immune systems were suppressed by treatment with cyclophosphamide 2 days after infection, splenomegaly was inhibited for about 6 days. However, when only humoral immunity was suppressed by treatment with cyclophosphamide 3 days before infection, splenomegaly still occurred. In addition, splenomegaly was absent in congenitally athymic nude mice. Nude mice and immunologically intact heterozygote mice were also sacrificed at varying times after infection, and spleens were examined histologically. A lymphocytic hyperplasia was observed in immunologically intact mice but not in nude mice. These data indicate that splenomegaly of T. musculi infections is T cell dependent and that splenomegaly is the result of a proliferation of B and/or T lymphocytes.