Glutaraldehyde has a wide spectrum of uses which can result in dermal contact with the agent. The low number of reports of hypersensitive reactions to glutaraldehyde indicates a low incidence of sensitization. This paper describes the contact hypersensitivity response to glutaraldehyde in the guinea pig and the mouse. Female albino Hartley strain guinea pigs and female B6C3F1 mice were sensitized with 0.3, 1.0 and 3.0% glutaraldehyde and challenged with 10% glutaraldehyde. Doses of glutaraldehyde were selected from assays for primary irritancy. Guinea pigs received 100 microliters by direct dermal application, for 14 consecutive days, and mice received 20 microliters by direct dermal application, for 5 or 14 consecutive days, to sites prepared by shaving and dermabrading. Rest periods were 7 or 14 days for guinea pigs and 4 or 7 days for mice. Measurement of the contact hypersensitivity response in guinea pigs was both visual evaluation (scoring) at 24 and 48 hours following challenge and radioisotopic assay at 48 hours, and in mice by radioisotopic assay 48 hours after challenge. Both guinea pigs and mice demonstrated dose-dependent contact hypersensitivity responses to glutaraldehyde. The radioisotopic assay appeared to be more sensitive than visual evaluation in detecting contact allergic hypersensitivity to glutaraldehyde.