The release pattern of excretory-secretory (E-S) products of Schistosoma japonicum eggs was investigated using eggs cultured in a chemically defined medium (MEMSE-J) for 16 days. The amount of protein released in culture supernatants was greater in 0- to 4-day and 12- to 16-day cultures than in 4- to 12-day cultures. The protein composition of E-S products and soluble extracts of newly laid eggs (N-SEA) and in vitro matured eggs (M-SEA) was analyzed by SDS-PAGE. Silver staining patterns of N-SEA and M-SEA were found to be similar except for the band at approximately 66 kDa, which appeared in highest concentrations in N-SEA. Western blot analysis with human infected sera showed antibody recognition of a 140- to 160-kDa antigen present in E-S products from mature eggs, while E-S products from immature eggs were unreactive. When either [35S]methionine or [3H]glucosamine was added to the culture medium, newly synthesized proteins or glycoproteins of the SEA and E-S products were labeled. Incorporation of both isotopes into SEA appears to correlate with developmental activity of the eggs. In contrast, release of E-S proteins and glycoproteins is more apparent as the miracidium matures. These results suggested that the source of E-S products from immature eggs is likely to be the collapsing vitelline cells and that of E-S products from mature eggs to be mainly miracidial secretions.