The immunological cross-reaction among sulfocephalosporins (cefsulodin, SFC-I, SFC-II), cephaloridine, sulbenicillin, penicillin G and 7-aminocephalosporanic acid has been investigated. A specific antibody was produced in rabbits against several structurally related haptens. The sensitization was conducted with a hapten-human serum albumin conjugate, which was prepared in an alkaline solution, with FREUND'S complete adjuvant. A crossreaction was clearly demonstrated between sulfocephalosporins and sulbenicillin, but not between sulfocephalosporins and cephaloridine, penicillin G or 7-aminocephalosporanic acid.