It is often considered that the inflammatory processes in the rheumatoid synovium are due to an immune response to an as yet unknown antigen; the resultant pannus has been referred to as" aggressive, invasive, proliferative and tumor-like." Data are presented which show that arthritogenic group A streptococcal cell walls stimulate human monocyte-macrophages to elevate the plasminogen activator (PA) activity of human synoviocytes. By analogy with other cellular systems, it is suggested that the monocyte-macrophage mediated elevation of synoviocyte PA activity can contribute to the tissue remodelling and cell migration found in the rheumatoid pannus. It is also proposed that the active moiety from the stimulated monocyte-macrophages is able to mimic, to some extent, the effects of transforming viruses and tumor promoters on cell function. This concept of a" tumor-like" synoviocyte transformation is suggested as being consistent with the descriptions of the rheumatoid pannus as" aggressive, invasive and tumor-like." Perhaps similar cellular interactions occur in other inflammatory diseases involving connective tissue turnover and cellular hyperplasia.