It has previously been shown that synovial fluid (SF) mononuclear cells (MNC) from patients with reactive arthritis (ReA) and some patients with undifferentiated oligoarthritis (UOA) respond specifically to the triggering bacterium (specific responders). However, in some patients there is a response to two or more bacteria (non-specific responders) and in a third group no response is found (non-responders). We assessed whether the proportion of synovial MNC which were macrophage-monocyte (MaMo) differed among the specific responder, non-specific responder and non-responder groups. There was no difference between the specific (33 +/- 9) and the non-specific (32 +/- 26) groups; non-responders had a higher percentage of MaMo (61.3 +/- 31%) although the difference was not significant. We also investigated whether the specificity of the response to antigen in ReA or UOA SF was altered by changing the T-cell/MaMo ratio. In all five specific responders the immune response remained specific whatever the ratio tested. However, four of the five non-specific responders, but none of the non-responders, developed a specific response to one of the tested antigens by increasing the T cell/MaMo ratio. We conclude that in some patients with a non-specific response, alteration of the T cell/MaMo ratio uncovers a specific response which may identify the triggering antigen.