Non-specific immunostimulants were used in an attempt to protect baboons from infection by schistosomes. Subcutaneous vaccination with cord factor (4.50 mg) and muramyl dipeptide (4.56 mg) 6 days before percutaneous exposure to 3000 Schistosoma haematobium cercariae/baboon (c.p.b.) failed to protect naive baboons: baboons with a 7-month-old, 5000 c.p.b. S. haematobium primary infection had developed too strong a natural immunity to detect any protection attributable to vaccination. Subcutaneous vaccination with 0.4 ml of Bacillus Calmette-Guerin (BCG, 1-8 X 10(8) colony forming units/ml) 4 days before exposure to 1000 c.p.b. S. mansoni gave a significant (38%) reduction in worm load compared with controls. However, vaccination with 0.8 (intramuscular) and 0.2 (intradermal) ml of BCG 11 days before exposure to S. mansoni 800 c.p.b. did not protect naive baboons, nor did it significantly reduce challenge worm recovery from baboons with a 13-week-old, 500 c.p.b. S. mansoni primary infection. Obvious pathology was seen at the site of vaccination in the first but not the second BCG experiment. These results partly support the findings in mice that non-specific macrophage and monocyte activators give partial protection against schistosome infections but they also illustrate that rodents and primates do not necessarily react identically. Hence, findings from rodent models should be extrapolated to man with some caution.