Peripheral blood lymphocyte mitogen responsiveness was studied in 21 patients with rheumatoid arthritis being treated with sodium aurothiomalate. There was a significant increase in lymphocyte response to concanavalin A and pokeweed mitogen but not to phytohaemagglutinin. This observed increase in lymphocyte response contrasts with the suppressive effect of gold salts in vitro. We propose that this apparent contradiction may be explained by the relatively low serum gold levels measured in our patients, compared with expected levels in synovial membrane. Thus gold could suppress rheumatoid inflammation in the "target tissue" while having little suppressive action in the peripheral blood compartment, where a removal of suppressive influences due to active disease might then be seen as a net improvement in lymphocyte responsiveness.