The inflammatory response to intradermal injections of urate, pyrophosphate and hydroxyapatite crystals in human forearm skin is described. Patients with rheumatoid arthritis responded normally to urate crystals, and patients with osteoarthritis or pyrophosphate arthropathy responded normally to hydroxyapatite and pyrophosphate crystals respectively. These results suggest that variation in host response to crystals cannot explain the different patterns of crystal-induced disease seen in man. The model, however, is recommended as a safe, simple ethical and reproducible test of inflammation in human subjects.