H uman blood mononuclear cells exposed to concanavalin A or phytohemagglutinin secrete a soluble factor that arrests the growth of human synovial fibroblastic cells in culture. Once the growth-inhibitory effect is initiated it cannot be reversed by washing the fibroblastic cells, by refeeding with nonconditioned fresh serum-containing medium, by trypsinization, EDTA treatment, or a combination of these procedures. Media from nonstimulated mononuclear cells, fibroblastic cells, or the lectins themselves do not contain similar inhibitory activity that can be detected by the present culture systems. This lectin-dependent, growth-inhibitory activity does not have a cytotoxic effect on the fibroblasts but increases their adhesiveness to plastic or glass surfaces, and the cells tend to assume a less fibroblastic morphology. The growth-inhibitory activity is stable in the cold and is nondialyzable or ultrafilterable, but the activity is rapidly lost at temperature between 60 degrees and 70 degrees C and at pH 2.0. The growth-arrested cells secrete more glycosaminoglycan per cell in the medium and synthesize more cell surface glycosaminoglycan than the controls. However, the increased glycosaminoglycan synthesis cannot be explained as being entirely secondary to a cell density effect as it is also observed when adjustments are made for the differences in growth rates.