Summary: Sheep were used to study the histology of normal lymphocyte transfer reactions and lesions that result from the interaction of sensitized cells with antigen. Reactions of the latter type were produced, either by the injection of antigen into the skin of sensitized sheep, or by the intradermal injection of sheep with spleen cells obtained from chickens that had been previously immunized with sheep tissues. The normal lymphocyte transfer lesion was found to be histologically distinct from the other dermal lesions. The most striking difference observed was the marked cellular proliferation that occurred in normal lymphocyte transfer lesions. The proliferating cells invaded and damaged blood vessels and other cellular structures present in the recipient's skin.