Lymphocyte blastogenesis during chronic schistosomiasis has been evaluated previously by either whole blood or peripheral blood mononuclear cell (PBMN) culture techniques. These two methods were compared in the present study. Blastogenesis to the mitogen phytohaemagglutinin was greater in the whole blood cultures at the time assayed. Substantial antigen-induced lymphocyte transformation was elicited in both culture systems using cells from uninfected patients. However, patients' responses to schistosome-derived antigenic preparations as determined by whole blood culture were greatly reduced compared with PBMN cultures. In view of the known immunoregulatory activities associated with sera and cells from infected patients the whole blood technique may better reflect in vivo lymphocyte reactivity. The PBMN cultures are better suited to examine the immunoregulatory activities which may develop during this chronic infection.