The rapid loss of bone mass is one of the serious problems which have to be solved before long-lasting manned spaceflights may be considered. In this paper we describe investigations in which we have checked whether the bone loss in astronauts as well as in osteoporotic patients may be related to abnormalities in a recently discovered calcium-binding protein, named osteocalcin. It was observed that in all subjects of a limited number of osteoporotic patients, the amount of calcium-binding groups (Gla-residues) in the circulating osteocalcin was substantially reduced. The Gla-content could be normalized, however, by the oral administration of vitamin K (1 mg/day). We also analyzed the Gla-content of plasma-osteocalcin from 4 astronauts before and after the D-1 mission. The amount of Gla-residues was reduced by more than 50% in the post-flight samples. It seems probable, that an increased vitamin K-intake by the astronauts will correct the observed abnormality, but whether this will lead to a decrease of the microgravity-induced bone-loss remains to be seen.