Suspensions of miracidia and cercariae of Schistosoma mansoni were subjected to repeated freeze–thaw cycles and then injected into resistant Salvador strain Biomphalaria glabrata snails. A pronounced increase in the number of mitotic figures, relative to uninjected, sham-injected, or diluent (water)–injected controls, was observed in the amebocyte-producing organ (APO) at 3 days postinjection (PI). After centrifugation of miracidia freeze–thaw extract (FTE), the resulting supernatant (FTS) and pellet possessed equal stimulatory activity that was approximately half that seen with FTE. Ultracentrifugation of miracidia FTS resulted in a supernatant that retained full activity, indicating a soluble molecule. Heat treatment of miracidia FTE reduced but did not eliminate activity, suggesting a nonprotein active component. Concentration or dilution of FTS by a factor of 10 gave a nonlinear dose–response relationship. Susceptible NIH albino snails injected with miracidia FTE had increased mitotic activity in the APO, which was much less than that seen in Salvador snails, whereas injection of miracidia FTE into Helisoma duryi had no discernable effect. Measurement of mitotic activity as a function of time PI showed no increase in numbers of mitotic figures in the APO at 18 hr but a large increase at 24 hr PI. Mitotic activity returned to preinjection levels by 96 hr PI, although a subsequent increase occurred at 120 hr PI.