The best therapeutic approach to acute schistosomiasis (Katayama fever) is still unsettled. In this paper we report a synergistic effect between schistosomicides and steroids in the treatment of the early stages of Schistosoma mansoni infection in the mouse. CBA mice infected with 150 S. mansoni cercariae were treated with oxamniquine or praziquantel and dexamethasone or prednisolone. The rate of parasite egg excretion by treated mice and appropriate controls was monitored, and the mice were perfused 43 d after infection for estimation of worm burdens and tissue egg densities. Mice treated with schistosomicides alone or with schistosomicides plus steroids had worm burdens of similar size. Significant reductions in egg counts were, however, recorded in faeces, and in the intestines and livers (with consequent reduction in liver pathology), of mice treated with schistosomicide and steroid, when compared to mice treated with schistosomicide alone or steroid alone. The apparent inhibition of fecundity of S. mansoni by combining these drugs has clear implications for treatment of the Katayama syndrome.