Some characteristics of hemagglutination (HA) by the BK virus, a new candidate for the papovavirus group, have been studied. Hemagglutinin prepared from cell cultures was found to be partially masked by inhibitors which could be dissociated from the virus by incubation at 37 C or by fluorocarbon extraction. Optimal conditions for HA are outlined. In routine tests, 0.5% human erythrocytes were used. The reaction was carried out at pH 7.0 on ice-water slurry. BK hemagglutinin receptors on human erythrocytes were found to be more resistant to neuraminidase than polyoma receptors. By gradient centrifugation analysis, two types of particles were found to be responsible for HA: (i) full, deoxyribonucleic acid-containing particles with a density of 1.325 g/cm(3) and (ii) empty capsids with a density 1.29 g/cm(3). Based on particle counting, one HA unit was calculated to correspond to 3 x 10(6) virus particles.