Lymphocyte typing (DLA typing) will be indispensable in the future for the transplantation research in dogs. For reliable cytotoxicity testing, a nearly pure suspension of fully viable lymphocytes is the most important prerequisite. As none of the methods described up to now in connection with DLA typing fulfills this criterion and as many of them are complicated and time-consuming, a number of modifications of the Ficoll-Isopaque technique were investigated. Of all methods tested, the immediate mechanical defibrination of the blood followed by centrifugation over a Ficoll-Isopaque gradient (mixture 24:10, density 1.077 g/ml) gave the best result. This method results in a yield of 50% of the lymphocytes with 100% viability, with only a few granulocytes, and virtually without contamination with erythrocytes, thrombocytes, and fibrin. The isolation can easily be terminated within 90 min, and it allows for simple and reliable lymphocytotoxicity testing.