Solubilized microsomes from bovine liver, kidney and testis were compared with regard to their content of vitamin-K-dependent carboxylase, the presence of endogenous vitamin K as well as that of endogenous carboxylatable precursor proteins. The isolation and purification of these protein substrates was not successful. Using antibodies against various well characterized proteins containing gammacarboxyglutamic acid (Gla), we were able to identify precursors of the blood coagulation factors II, IX and X in liver microsomes. The nonhepatic proteins could not be identified in this way. Gla-containing proteins, however, were isolated from human sperm, urine and renal stones. It was demonstrated that - like osteocalcin - also the urinary Gla protein inhibits the precipitation of various calcium salts from supersaturated solutions. The concentration of the urinary Gla protein (16 mg/l) in human urine is well above the concentration required for the in vitro inhibition of salt precipitation.