Of 6 patients with penicillin reactions, 3 whose reactions were severe were found to possess the "L. E." factor in the plasma and "L. E." cells in the bone marrow. The 3 who failed to show the "L. E". phenomenon had only minimal reactions to penicillin. Since the group studied was small, no conclusions regarding the incidence of the "L. E." phenomenon in penicillin reactions can be drawn. The findings are discussed in the light of the possibility that the phenomenon may be related to hypersensitivity reactions as well as to systemic lupus erythematosus.