The relationship of vaccinia haemagglutinin (HA) to extracellular enveloped virus (EEV) was examined. EEV banded in caesium chloride gradients at a density of 1.23 to 1.24 g/ml coincident with a peak of HA activity. EEV of an HA+ vaccinia strain showed greater than 90% adsorption to rooster red blood cells (RBCs) as detected by infectivity and 3H-thymidine labelling whereas intracellular naked virus (INV) of the HA+ strain and EEV of an HA- strain failed to show significant adsorption. The adsorption was specifically inhibited by antiserum to vaccinia. Adsorption kinetic experiments demonstrated a lack of temperature dependence on the total amount of EEV adsorbed. No elution of EEV from RBCs could be detected. The capacity of EEV to adsorb to RBCs was found to be stable at 56 degrees C for 30 min.