In previous studies (Young and Podesta, 1984, Journal of Parasitology 70: 879-885, 931-936), 5-HT (5-hydroxytryptamine), and the C3b component of complement were shown to increase the synthesis of phosphatidylcholine (PC) from choline in adult Schistosoma mansoni. Since the authors were primarily interested in surface membrane synthesis (the major phospholipid of which is PC), the present series of experiments were conducted to examine the effect of C3b and 5-HT on surface signal transduction, leading to the synthesis of the apical plasma membrane and overlying envelope of S. mansoni. Cercariae of S. mansoni were shed from infected Biomphalaria glabrata obtained from the University of Lowell Research Foundation, Lowell, Massachusetts. The cercariae were injected into Syrian hamsters (Charles River, St. Constant, Quebec, Canada; 2,000 cercariae per animal) or were transformed into schistosomula by penetration through excised hamster skin (Clegg and Smithers, 1972, International Journal for Parasitology 2: 79-98). Infected hamsters were necropsied at 45 days post-infection and were perfused with ice cold Krebs Ringer Phosphate Buffer (KRP pH 7.4) to remove the adult worms. Approximately 1-2,000 adult worms or 10,000 schistosomula were incubated for 1 hr at 37 C in Eagle's Minimal Essential Medium (MEM) containing 10 itCi/ml [methyl14C] choline chloride (sp. act. 55 mCi/mmol, New England Nuclear) with the experimental incubations containing 520 nM 5-HT (Sigma, St. Louis, Missouri) or complement components consisting of 125 Ag C3 and 30 ,tg C4 (Kallestad, Montreal, Quebec, Canada). Schistosomula were washed briefly in MEM, homogenized in KRP, and protein was determined by the method of Bradford (1976, Analytical Biochemistry 72: 248-254). The apical plasma membrane and outer bilayer of the apical bilayer complex of the adult parasites were separated by the method of McDiarmid et al. (1982, Molecular and Biochemical Parasitology 7: 141-157). Lipids were extracted, identified, quantified and the amount of labelled choline incorporated determined as described previously (Young and Podesta, 1982, Molecular and Biochemical Parasitology 5: 165-172). The data are presented as the means ? SEM. The major phospholipids of the apical bilayer complex are phosphatidylcholine, 1.1 + 0.1 (n = 3) mg/mg protein in the plasma membrane and 2.6 ? 0.2 (n = 3) mg/mg protein in the outer bilayer; phosphatidylethanolamine, 0.4 + 0.2 (n = 3) mg/mg protein in the plasma membrane and a trace in the outer bilayer; total phospholipid, 1.7 ? 0.3 (n = 7) mg/mg protein in the plasma membrane and 2.8 ? 0.3 (n = 7) in the outer bilayer. Cholesterol was 0.6 ? 0.1 (n = 3) mg/mg protein in the plasma membrane and 1.3 ? 0.6 (n = 3) in the outer bilayer. In schistosomula, 5-HT increased choline incorporation into phosphatidylcholine from 4,470 + 80 (n = 4) to 40,800 ? 1,100 (n = 3; significantly increased P < 0.05) pmols phosphatidylcholine/mg protein/hr. The addition of C3 to the incubation medium did not affect choline incorporation into phosphatidylcholine in schistosomula (4,650 ? 70 (n = 3) pmols phosphatidylcholine/mg protein/hr). The addition of both 5-HT and C3/C4 to the adult incubations increased choline incorporation into phosphatidylcholine in the apical bilayer complex (Table I). Choline incorporation into the plasma membrane was increased almost 6-fold by 5-HT, but was unaffected by C3/C4. Choline incorporation into phosphatidylcholine in the outer bilayer was unaffected by 5-HT but was increased 4-fold in the presence of C3/C4 (Table I). It is interesting that C3/C4 does not affect choline incorporation in schistosomula as it would be available in vivo to both the schistosomulum and the adult parasite. The failure of schistosomula to respond to C3/C4 and the absence of the