The effects of non-lethal amounts of a variety of pore-forming agents on cultured human rheumatoid synovial cells (HRSC) have been investigated. Non-lethal complement membrane attack and non-lethal amounts of melittin, perforin and ionomycin all caused a biphasic release of prostaglandin E2 (PGE2) from HRSC, an early phase of release occurring within 1 hr and a second larger phase commencing after 4 hr and continuing over the 24-hr time-course. Removal of extracellular calcium abolished the release of PGE2 under all conditions of non-lethal attack. Modulation of G-protein activity reduced the second phase of release caused by non-lethal doses of the membrane-attack complex (MAC) from 800 ng/10 (6) cells PGE2 to around 300 ng/10 (6) cells. Non-lethal levels of the MAC also caused release of interleukin-6 (IL-6) from HRSC over the 24-hr time-course, with levels reaching 550 ng/10 (6) cells at 24 hr compared to background levels of 200 ng/10 (6) cells. No detectable release of IL-1 alpha could be measured at any time following non-lethal complement membrane attack. These results suggest a role for the MAC as an initiating mediator inducing the inflammation associated with rheumatoid arthritis.