Complement-fixing antibodies to the heat-inactivated keratinase II of Trichophyton mentagrophytes var. granulosum were detectable in sera from approximately 40% of the Hartley strain guinea pigs previously infected with this dermatophyte. However, no precipitating antibodies to the heat-inactivated keratinase II were detectable in these sera. Guinea pigs immunized with the heat-inactivated keratinase II had both complement-fixing and precipitating antibodies to the keratinase II. Circulating antibodies in sera from both the infected and the immunized guinea pigs also reacted with the active keratinase II but did not inhibit its proteolytic activity.