Abstract: The effects in vitro of schistosomula- and adult worm-derived metabolic products on the proliferative response of splenic lymphocytes to soluble egg antigen (SEA) in mice infected with S. japonicum was studied. The results showed that the proliferative response of splenic lymphocytes to SEA in infected mice was enhanced when schistosomula-derived metabolic products were added to the in vitro culture of splenic lymphocytes and SEA (Fig. 1). However, adult worm-derived metabolic products had inhibitory effects on the proliferative response in the above mentioned culture (Fig. 2). It was suggested that there were certain regulative effects of schistosome-derived metabolic products on the proliferative response in mice infected with S. japonicum in vitro, and these regulative effects varied with the factors released by different developmental stages of schistosomes.