Forty-one schoolchildren with positive stools for Schistosoma mansoni eggs and 39 age- and sex-matched children with negative stools were given 3 doses, 5 µg each, of a plasma derived hepatitis B vaccine. Their sera were examined 3 and 9 months after the third dose for hepatitis B surface antibody (anti-HBs). At 9 months after vaccination, both the number of responders and the mean of antibody titers were significantly higher in the control group than in the group infected with S. mansoni (97% vs. 56% and 334.8 ± 192.9 vs. 67.7 ± 74.4 mIU/ml). There was a negative correlation between anti-HBs titers and the long diameter of the spleen as well as between the titers and the long diameter of the spleen and the liver span in the mid-clavicular line (right lobe) taken together. There was also a positive correlation between anti-HBs titers and the diameters of the portal vein. There was no correlation between anti-HBs titers and any of the followin 3 parameters: liver span in the mid-clavicular line (right lobe) alone, liver span in the middle line (left lobe) alone, and the degree of thickness of the periportal fibrosis. There was also no correlation between anti-HBs titers and egg counts in the stools. An interpretation of these findings was made in the light of the role of phagocytic activity of the liver and spleen as well as of porta-caval shunts in the immune mechanism.