To provide rapidly phenotyped units of blood, we adapted the hexadimethrine bromide (Polybrene) technique to microplate technology. Pilot samples from 282 donor units were phenotyped for antigens in the Rh, Kidd, Kell, Duffy, and Ss systems with a standard tube-testing method and a Polybrene-microplate (P-MP) technique. Diluted antisera and a 1 percent red cell suspension were used to give P-MP reactions that were accurate and easy to interpret. One microplate, containing 96 tests, was prepared and read within 15 minutes for P-MP tests yielding direct agglutination (Rh, Jkb, Fya, Fyb), or within 19 minutes for P-MP tests requiring an antiglobulin phase (Jka, K, S, s). No false-positive results were found. No false-negative reactions were found in typing for Rh, K, S, s, or Jka antigens. Phenotyping for Jkb and Duffy antigens gave a false-negative rate of less than 0.015. Considerable savings in reagents were obtained through microplate miniaturization and through the enhancement of apparent antibody avidity in the Polybrene/low ionic medium which permitted dilution of reagent antisera. The P-MP technique affords a rapid, accurate, simple, and inexpensive means of phenotyping large numbers of donor units.