Observations made on a white man at irregular intervals for 4 weeks suggested that he possesses a substance, presumably associated with his sweat, which is both toxic and repellent to ticks. Tests in which he held adults of Amblyomma americanum and nymphs and adults of Dermacentor andersoni in the hand or in the antecubital space showed that adults of both spp. were immobilized in 6-40 min. and nymphs were, as a rule, killed in 3-10 min. Revived ticks exhibited mild akinesis and uncoordinated movements. The results of tests in which ticks were confined in-cells on the skin of the upper arms and thighs, i.e., where there was no free perspiration, were inconsistent, the ticks not always being affected. Fresh urine from the subject was tested against ticks with negative results. There was no evidence that fraud, trickery or any environmental factor was concerned in the findings, and information obtained relative to the subject''s medical and personal history did not appear to have any bearing on observed phenomenon.