Statistical assessment has been made of synovial mononuclear cell responses to ureaplasma, chlamydial, enterobacteriaceae and mumps antigens in 16 cases of sexually transmitted Reiter's syndrome, 8 cases of enteric Reiter's syndrome and 12 cases of rheumatoid arthritis. These 3 groups of patients can be differentiated by synovial mononuclear cell responses to these microbial antigens and the microbiological origin of the arthritis in Reiter's syndrome can probably be suggested by this investigative approach.