Metronidazole administered orally in doses of 20 and 200 mg/kg daily suppressed granuloma formation around Schistosoma mansoni eggs which were injected intravenously and lodged in the pulmonary microvasculature of mice. The same doses did not suppress granuloma formation in animals which had previously been sensitized to the eggs. Nonspecific granulomatous inflammation around divinyl benzene copolymer beads was unaffected by the drug. In a daily dose of 20 mg/kg, metronidazole inhibited delayed footpad reactions to soluble schistosome egg antigen, but 200 mg/kg on alternate days failed to suppress skin allograft rejection. The drug appears to suppress selectively some aspects of cell-mediated immunity.