During 9 experimental and 93 clinical operations on the open heart under circulatory bypass the authors studied microclots of multicomponent perfusion mixtures. They observed an increased number of microclots after infusion of high-molecular plasma-substituting solutions and long-term intraapparatus recirculation. During the bypass there was an increased number of microclots in the "machine-patient" system. The complex of measures suggested by the authors contributes to a delay in additional formation of microclots during the perfusion and also reduces the number of posttransfusion complications.