With the intent of providing a useful tool for studies on the cellular and molecular biology of Schistosoma mansoni, we have attempted to establish indefinitely proliferating cell lines. Primary (mother) sporocysts have served as sources of tissue fragments for initiation of primary cultures in complex media containing fetal bovine serum. Viability is maintained for several months, during which time there is differential survival of individual cell types. Cells that ultrastructurally resemble germinal cells are among the most persistent. Contractile responsiveness to serotonin and flagellar movement of flame cells are sustained for several weeks. Exposure to epidermal growth factor failed to induce tyrosine phosphorylation as detectable by western blot analysis. Incorporation of 5-bromo-2'-deoxyuridine into nucleic acid has been used as an indicator of the merit of experimental variables tested for their growth-promoting potential. Continuous proliferation remains an elusive goal, but coculture with host snail ganglia has yielded promising results. These primary cultures can be used to obtain useful information on parasite physiology. In light of our results, and of the varied lines of investigation that would be facilitated by such tools, further efforts to immortalize cell lines are warranted.