The effects of interleukin-1 alpha and beta were tested on the [3H]-arachidonic acid release and the prostaglandin synthesis by human cultured synovial cells and chondrocytes. Both forms of interleukin-1 stimulated the arachidonic acid release but interleukin-1 beta was more potent than IL-1 alpha. Human synovial cells and chondrocytes synthesized three types of prostaglandins upon stimulation with interleukin-1 alpha or beta: prostaglandin E2, F2 alpha and 6-keto-prostaglandin F1 alpha. Regarding the synthesis of these prostaglandins, IL-1 beta was again more potent than IL-1 alpha. A comparison between interleukin-1-stimulated synovial cells and chondrocytes revealed neither significant quantitative nor qualitative differences in both the arachidonic acid release and the prostaglandin synthesis.