BM 12.531, 2-[2-cyranaziridinyl-(1)]-2-[2-carbamoylaziridinyl-(1)]-propane, a new immunostimulant compound, increased the resistance of mice to infection with Candida albicans. Because BM 12.531 had no fungistatic activity in vitro, it is proposed that the therapeutic effect of BM 12.531 is caused by the stimulation of cell-mediated immunity. Administration of cyclophosphamide alone increased the mortality among mice infected with C. albicans and Pseudomonas aeruginosa, but when BM 12.531 was then administered to these animals, the mortality was reduced. Among mice with acute Escherichia coli infection, a synergistic effect of chloramphenicol and BM 12.531 was demonstrated.