Summary A steroid, 6-chloro-17-hydroxy-pregna-l,4,6-triene-3,20-dione (CHP), that exhibits selective activity in several models of cellular immunity including an apparent inhibitory action on the elicitation of delayed hypersensitivity, was examined in a new, simple experimental model for assessing aspects of host cell-mediated immunological competence. This model is based upon the capacity of the adult mouse to prevent the progressive growth of tumors induced by the Moloney sarcoma virus. Two steroids reported to have immunosuppressive activity in other assay systems, namely, cortisol and progesterone, were also studied. Control mice and those injected with CHP maintained their capacity to reject the tumor. In contrast, significant numbers of mice receiving a single large injection of cortisol or progesterone succumbed to progressive tumor growth under the experimental conditions used. The data indicate that CHP, while influencing selected parameters of cellular immunity, e.g., the elicitation of delayed hyper-sensitivity, does not decrease the capacity of the host to mount a defense against the progressive growth of the Moloney virus-induced sarcoma. The results indicate that CHP may be useful in modulating specific aspects of cellular immunity without altering others. In addition, the experimental model described provides a simple method of assessing the possible immunosuppressive effects of naturally occurring and synthetic agents on viral-induced tumor growth. The authors wish to thank Dr. Gary Thurman for assistance with the statistical analysis.