Studies on antigenicity of suloctidil were performed on ASA, Schultz-Dale reaction, PCA and PHA in guinea pigs. Two sensitizing procedures were enforced. One was performed by means of treatment of suloctidil (p. o. and i. p.) and the other was done by means of injection of suloctidil, suloctidil-BSA-mixture and suloctidil-BSA-conjugate emulsified with FCA. Guinea pigs treated with suloctidil by oral or intraperitoneal administration showed no ASA, Schultz-Dale reaction, PCA and PHA by the challenge of suloctidil. The animals treated with suloctidil-BSA-conjugate only evoked severe anaphylactic reaction to challenge of suloctidil-OVA-conjugate. Guinea pigs treated with suloctidil and suloctidil-BSA-mixture showed no anaphylactic reaction to challenge of suloctidil, suloctidil-OVA-mixture and suloctidil-OVA-conjugate. The animals sensitized with suloctidil-BSA-conjugate showed no anaphylactic reaction to challenge of suloctidil and suloctidil-OVA-mixture. Therefore, it is concluded that suloctidil does not have any antigenicity.