Skin reactivity to tuberculin has been studied during the course of experimental leptospirosis in guinea pigs. A depression of the delayed hypersensitivity to tuberculin was demonstrated in the infected animals. The depression was most pronounced when icterus had developed. The depression was not correlated with the amount of infectious units administered or with the demonstration of live leptospirae in the peritoneal cavity. In the infected animals there was no correlation between the initial and the final skin tests which is in contrast to findings in the control group.