ZIMET 3164 inhibited the growth of sarcoma 180 P, sarcoma 180 G, and Walker 256 carcinosarcoma, but was unable to prolong the survival time of mice bearing Ehrlich ascites carcinoma or the leukaemias L 1210 and LAJ I to a worthwhile extent. The primary and secondarantly suppressed in mice. The drug exerted maximum effect when given on days--2 to +2 relative to antigenic stimulus. Administration exclusively prior to immunization induced only moderate immunosuppression while injection afterwards failed to affect the primary response at all, suggesting that the drug interfers with the afferent limb of immune response. In general, ZIMET 3164 proved to be half as effective as cyclophosphamide, but more effective than chlorambucil.