Inbred strain 2 and random-bred guinea pigs injected oxazolone in incomplete or complete Freund's adjuvant showed contact reactions within an hour after topical application when tested 3 weeks post-sensititization. The application when tested 3 weeks post-sensitization. The time for appearance of skin reactions at 5 days post-sensitization was 8 to 24 hr, more along classic delayed-type hypersensitivity lines. Immune serum harvested several weeks after sensititazation was effective for passive transfer of early skin reactivity, while serum at 5 to 10 days was not. Lymph node cells taken 5 days after sensitization of donors could transfer only the late skin reactivity to histocompatible recipients; lymph nodes taken at 15 days did not contain cells capable of such transfers. The heterogeneity of mechanisms involved in the production of contact dermatitis suggested by these results provides new approaches to studies in this area of allergy research.