A single intra-articular injection of lymphokine into the guinea-pig knee joint resulted in a sequence of changes in joint architecture whose histopathological features resembled that of an acute inflammatory reaction progressing to a chronic state. At 24 h there was a mild hyperplasia and hypertrophy of the synovium with intense polymorphonuclear leucocyte infiltration. At 72 h, the synovium was heavily infiltrated with diffuse and focal aggregations of mononuclear cells; erosion of cartilage and bone by synovial pannus was accompanied by a subsynovial fibrosis. By 1 week, leucocytic infiltration of the synovium had decreased markedly although the erosion and fibrosis persisted. However, when lymphokine was injected together with oil-elicited peritoneal exudate cells a more intense arthritis ensued: at 72 h synovial pannus was prominently eroding bone and this was accompanied by the appearance of multinucleate cells resembling osteoclasts in the zone of erosion. These features were shown to resemble closely the histopathology of experimental allergic arthritis in the guinea-pig, in contrast to the lesser severity of synovitis resulting from the adoptive cellular transfer of delayed hypersensitivity into the joint. The results indicate that lymphokines may play a role in the induction of experimental allergic arthritis by recruiting and activating cells involved in chronic inflammation.