In this study which included 16 patients with inflammatory or non-inflammatory arthropathies, human granulocyte-macrophage colony-stimulating activity was detected in synovial fluid. This was attributable to the presence of colony-stimulating factor(s) (CSF), as a direct action on human bone marrow progenitor cells was demonstrated using clone transfer experiments. Samples of synovial fluid also stimulated the growth of murine macrophage colonies and induced differentiation in the murine myelomonocytic leukemia cell line, WEHI-3B(D+), which are characteristic properties of human macrophage-CSF or granulocyte-CSF respectively. These findings and the results of preliminary fractionation procedures suggested that the colony-stimulating activity in synovial fluid was not explicable by the presence of any one of the well-characterized human CSF acting in isolation. This provides a new insight into the pathogenesis of inflammatory arthropathies and supports the hypothesis that CSF have important roles in vivo in addition to the regulation of haemopoiesis.