A method is described for rapid and efficient separation of leukocytes and nucleated erythrocytes from blood or marrow. It is based on the rediscovery of a nontoxic hemagglutinin, isolated from common red or navy beans, which agglutinates all human erythrocytes and those of the animals which have been tested with it. The time from drawing the blood to complete separation of the cells is less than ten minutes. The cells remain in suspension in their own plasma and are countable. Negligible amounts of foreign material are introduced, a great advantage in chemical studies. The cells so isolated are living and suitable for culture studies. Any volume of blood from less than 1 ml. to over 500 ml. may be processed. In marrow nucleated erythrocytes are separated with the leukocytes and in their original porportions. The volume, motility, morphology and life span of the cells in cultures are not altered by addition of the bean extract.