Immunological properties of cefpirome sulfate (CPR) were examined. The immunogenicity and challenging ability of CPR were examined in guinea pigs by active systemic anaphylaxis (ASA) and homologous 4-hr passive cutaneous anaphylaxis (PCA) tests. The animals given CPR alone intraperitoneally for sensitization and their sera were negative for ASA or PCA reactions, like the results with reference substances, ceftazidime (CAZ) and bephalothin sodium (CET). When each antibiotic plus Freund's complete adjuvant (FCA) was used for sensitization, ASA reactions were observed with CPR, cephaloridine (CER), CET, and cefazolin sodium (CEZ), and PCA reactions, with CPR and CET. CPR had the ability to challenge the ASA and PCA reactions. CER and CET also showed the ability to challenge ASA or PCA reactions, though at low incidences. The cross-reactivity of CPR with commercially available antibiotics was examined by heterologous PCA test and by passive hemagglutination test and its inhibition test. The antiserum used was from rabbits immunized with each antibiotic-ovalbumin conjugate plus FCA, and the antigen was each antibioticbovine serum albumin conjugate. CPR cross-reacted markedly with cefotaxime sodium (CTX) having the same side chain at position 7 and showed weak, unidirectional reactions with CAZ and CET. In the in vitro direct Coombs test, the positive reactions noted with CPR were stronger than those with latamoxef sodium, equal to those with CEZ and slighter than those with CTX, CET and benzylpenicillin potassium. In conclusion, in the safety evaluation of CPR, its antigenic potential may not be a problem, like the cases of other antibiotics.