The synovial fluids of patients with a destructive form of osteoarthritis (DOA) were shown to contain high levels of bone resorbing activity as judged by the ability of the fluid to stimulate the release of 45Ca from labelled cultured mouse calvariae. The activity was lost on extended storage of the synovial fluids and was dependent for its effect on cellular activity in bone. Bone resorbing activity was present in most synovial fluids from patients with DOA and rheumatoid arthritis (RA) but occurred at higher levels in the former. In contrast, interleukin 1 (IL1) activity, measured by the mouse thymocytes costimulation assay, was higher in RA than DOA synovial fluids. Little or no bone resorbing or IL1 activity was detected in synovial fluids from patients with pseudogout or non-destructive osteoarthritis. These results suggest that most DOA synovial fluids contain a bone resorbing factor other than IL1. It is considered that the factor may be produced by synovial cells stimulated by hydroxyapatite crystals.