Abstract. It appeared from our previous studies that Ca2+ concentrations comparable to those occurring in blood protect the C‐terminal part of the γ‐chain remnants of fibrin(ogen) fragments D against further plasmin attack. This study shows that purified fibrin(ogen) fragments D formed in the presence of Ca2+ ions have anticlotting activity in vitro, whereas shortening of the γ‐chain remnants leads to loss of such activity. Copyright © 1979, Wiley Blackwell. All rights reserved