We found previously that crystals of sodium urate and silicon dioxide (silica) can stimulate the production of endogenous pyrogen (EP), now called interleukin-1 (IL-1), the polypeptide mediator of fever and other aspects of inflammation. We have confirmed and extended the work with urate crystals and have examined 2 other crystals associated with joint problems, hydroxyapatite (HA) and calcium pyrophosphate dihydrate (CPPD). The crystals were added to suspensions of human blood leukocytes (2.5 X 10(6) monocytes/dose, with 10% fresh autologous plasma); after 18 hours of incubation, the EP content of the supernatants was assayed in the rabbit pyrogen test. HA and CPPD crystals neither induced EP production nor reduced the amount of staphylococci-induced EP. Presized (10 - 40 micron) urate crystals were pyrogenic, but less so than the unsized and aggregated urate crystals investigated previously and reexamined here. On ultrasonication, the aggregated urate crystals became first more pyrogenic and then less so as the crystals were dispersed and broken down. Ultrasound did not impart pyrogenicity to HA or CPPD crystals: their failure to stimulate EP/IL-1 production from leukocytes in vitro indicates a difference in their phlogistic properties, compared with crystals of urate or silica. The results with urate crystals have pathogenetic implications in a number of areas of gouty inflammation: initiation of the acute attack, other aspects of the acute-phase response, polyarticular involvement, and the inflammatory consequences of chronic stimulation by tophaceous material.