Adverse reactions produced by sulfhydryl compounds with active thiol groups have generally formed a distinctive pattern in man when they are viewed as a class. It has been reported that cutaneous SH-induced drug eruptions have a wide variety of clinical presentations; histologically, they show a pattern of eosinophilic necrosis and/or satellite necrosis similar to that seen in cutaneous graft vs host reactions. In the present experiments, guinea pigs were sensitized with cephalothin (CET) and thiol compounds such as tiopronin (TP), D-penicillamine, gold sodium thiomalate or thiomalate, using a method similar to that described previously. In lymphocyte stimulation tests, lymph node cells from the sensitized animals responded positively to spleen cells pulsed with each thiol compound. Intracutaneous tests revealed some positive reactions to each thiol compound; there was a tendency to produce a tuberculin type reaction with indurated erythema rather than the Jones-Mote type seen in CET-induced reactions. The dose-requirements for positive intracutaneous tests and generalized rash (GR) due to thiol compounds were lower than for CET, which required relatively large doses. Histologically, infiltration of basophilic cells was prominent in the skin lesions induced by intracutaneous tests with CET and in those of CET-induced GR. On the other hand, intracutaneous tests with TP following the induction of TP-induced GR revealed eosinophilic degeneration of epidermal cells, which was similar to the eosinophilic necrosis seen in cutaneous GVHR. Intracutaneous tests after the induction of CET-GR did not show any eosinophilic changes in the epidermal cells. These findings are reminiscent of the characteristics of eruptions induced by thiol compounds in man, which differ from the eruptions induced by CET.