SUMMARYThe relative efficacy of azathioprine, chlorambucil, methylprednisolone, and total body γ-radiation, alone and as combined therapy, in suppressing tetanus antitoxin production in mice was determined. An in vivo “live-die” assay for serum antitoxin titer was developed. Increasing the number of drugs in the treatment regimen to two or three caused greater depression of the primary immune response than did only one drug. The most profound immunosuppression was produced when sublethal radiation and three drugs were used together