Local exposure of the sites of elicitation (ears) of contact hypersensitivity (CHS) in mice to relatively modest doses of ultraviolet radiation (FS40 sunlamps) daily for 4 days prior to challenge in previously immunized animals results in significant enhancement of the CHS response as measured by 24-h swelling. This effect was seen in both C3H/HeJ and A/J mice and with the use of both trinitrochlorobenzene and dinitrofluorobenzene as contact sensitizers. Doses of PUVA (parenteral administration of 8-methoxypsoralen followed by exposure to UVA, 320-400 nm, radiation) up to that which can systemically suppress the induction of CHS failed to affect the elicitation of CHS. Irritation of the sites of elicitation of CHS by applications of turpentine also failed to affect the CHS elicitation reaction. Enhancement of the CHS elicitation response by local ultraviolet radiation exposure prior to challenge appears to be a specific photobiologic event.