A total of 29 rheumatoid patients and 19 nonrheumatoid patients were tested for evidence of present or past infection b y M pneumoniae, M hominis, M fermentam, M arthritidis, M pulmonis, and M hyorhinis. T h e techniques of lymphocyte transformation, metabolic-inhibiting antibody test, and mycoplasmacidal antibody test indicated no significant difference in the response of rheumatoid as opposed to nonrheumatoid patients.Although a wide variety of immunologic processes are apparent within the joints of patients exhibiting rheumatoid arthritis (RA), the nature of the initiating agent or event appears elusive. An infectious etiology for human rheumatoid arthritis remains an attractive hypothesis despite numerous conflicting and inconclusive results on the isolation of an agent.