Using a whole blood culture technique, lymphocyte responsiveness was assayed as 3H thymidine incorporation in 41 individuals infected with Schistosoma japonicum. In comparison with controls, response to phytohemagglutinin (PHA) was suppressed but was unrelated to egg excretion levels or duration of infection. Reactivity to whole worm antigens was significantly greater in individuals with a higher level of egg excretion and presumably greater infection intensity. The response to egg antigens significantly decreased with age and duration of infection. Some individuals with a depressed responsiveness to PHA maintain a high reactivity to parasite antigens, suggesting that different subpopulations of lymphocytes may be involved in these measurements.