The T and B lymphocyte composition of the lymphoid organs, peripheral blood, and hepatic granulomas was determined in mice lightly infected with Schistosoma mansoni. Apart from an increase of circulating B cells, no change was seen in the distribution of lymphocytes prior to oviposition. Thereafter (8-20 weeks), a pronounced trend toward increased B and increased T cell percentages occurred throughout the organs. This effect was largely due to marked increases in the B cell population which outweighed increases of T cells occurring at 8 and 16 weeks. By the late chronic period (32 weeks), an overall normalization of percentages was observed due to declining B and/or increasing T cell numbers. Hepatic granulomas also showed notable compositional changes. At the time of maximum granulomatous response (8 weeks), the lymphocyte population of these lesions consisted primarily of T cells. Subsequently, during the time of modulated granuloma formation (12-32 weeks), B-cells became a significant component, comprising 10% of the granuloma cell population. The appearance of B cells within granulomas may indicate that they play a role in modulating granulomatous hypersensitivity.