This investigation represents definite progress in the field of assisted circulation. Experiments involved the maintenance of partial left ventricular bypass in 25 calves by means of a new, implantable, left ventricular assist pump. Flow rates of 800 to 3,000 ml/ min were employed continuously for periods of 6 to 120 days. Flocked Dacron fibrils were applied to all pump surfaces to serve as a matrix for adherence of a fibrin-cellular (pseudoendothelial) layer. Serial hematological studies carried out during assisted circulation did not disclose any evidence of severe blood damage.