The transformation of free-living cercariae to parasitic schistosomula of Schistosoma mansoni is accompanied by the formation of a multilaminate surface membrane which binds concanavalin A (Con A). This transition normally occurs with penetration of intact skin of the mammalian host, but can be accomplished by vortexing and incubating organisms at 37°C in complex tissue culture medium (e.g., Earle's lactalbumin). We determined the minimal conditions that initiate surface maturation by monitoring surface changes by transmission electron microscopy and surface fluorescein-conjugated Con A labeling. Cercariae incubated in water for 2 hr 2 hr at 37°C retained their trilaminate surface and were non-fluorescent. After incubation in 116 mM NaCl for 3 hr at 23°C, no organisms were covered by intense surface fluorescence; increasing the temperature to 37°C resulted in 50% of the parasites becoming fluorescent. The addition of 26 mM NaHCO3 (pH 7.4) to parasites incubated in NaCl at 37°C resulted in 98% of the organisms binding Con A. No significant difference in surface Con A binding was found between organisms incubated with the chelators EGTA or EDTA and controls. Mechanical stimulus by itself was not effective in stimulating surface maturation. The conditions that initiated surface maturation were elevation of temperature to 37°C and NaHCO3-containing medium.