Patients with severe nodulo-cystic acne are known to have elevated serum antibody levels and increased immediate hypersensitivity reactions to Propionibacterium acnes. This organism is the predominant bacterium in normal pilosebaceous follicles of human skin, and can be consistently isolated from pustular lesions in acne. Previously it had been observed that delayed cutaneous hypersensitivity reactions to P. acnes were negative in patients with acne. The present study investigated the proliferative response of lymphocytes from patients with nodulo-cystic acne to phytohaemagglutinin (PHA) and P. acnes antigen stimulation. The response to PHA stimulation was within normal limits. The response to P. acnes antigen showed a significant increase over control values obtained by testing lymphocytes from acne-free subjects. Thus cell mediated immunity to P. acnes may be present in subjects with severe inflammatory acne. These findings raise the possibility that reactions to P. acnes may contribute to intensifying the inflammatory response in acne lesions.