The nature, prevalence, and specificity of birefringent calcific particles in granulomas of sarcoidosis have been examined, including histochemical reactions, single particle, and microchemical analyses. Particular attention was paid to small ovoid forms of which most were calcium oxalate monohydrate. Larger crystals, those within giant cells, and the birefringent component of a Schaumann complex were also calcium oxalate. Small ovoids appeared to originate in macrophages and to be precursors of other forms; they were found in 86% of lymph nodes and 73% of surgical lung specimens. They were not specific for sarcoidosis. Organisms could not be certainly identified in them. Their origin is discussed in relation to activated macrophages, calcium, and oxalate metabolism, and the role of calcium oxalate in granulomas is considered. Four particles from two cases were dolomite and two were a calcium-sulphur compound. The biologic origin of dolomite is reviewed.