The myeloid cell line K562 was used to screen for specific antibodies in sera obtained from pregnant women and multitransfused patients. The alloantisera thus procured reacted with random granulocytes in a pattern suggestive of a polymorphic system. Some antibodies were found to react only in the cold. A simple reproducible method for the simultaneous separation of granulocytes and lymphocytes for microcytotoxicity is described.