A hemagglutination test using coated sheep erythrocytes was devised for quantifying circulating antibodies directed against an extract of sarcoid spleen (K veim suspension). Out of 361 patients with respiratory sarcoidosis, the titres for 336 were less than 1:1,280. In 335 patients with various disorders the titres were less than 1:1,280 in 29. In 151 healthy controls the titres were never found to be less than 1:160. The circulating antibodies seemed to react specifically with antigenic determinant(s) contained in the K veim suspension and belonged primarily to the IgM class.