The spontaneous binding of group A streptococcal lipoteichoic acid (LTA) to mammalian cell membranes was studied in isolated membranes of human erythrocytes. The binding of radiolabeled LTA to erythrocyte membranes was dependent on membrane concentration and time. Binding approached a maximum within 30 min of incubation. The bound LTA could be displaced by adding a 50-fold excess of unlabeled LTA. The displaced LTA was eluted from a column of Sepharose 6B in a position identical to that of authentic LTA, suggesting that binding did not alter the size of the molecule. A dissociation constant of 42 micrometers was calculated, and only one population of approximately 5.5 X 10(6) binding sites per erhtyrocyte membrane was detected. Since these results suggested that erythrocyte membranes possess specific binding sites for LTA, an attempt was made to localize the putative receptors to the outside or the inside surface of the erhtyrocyte membrane. Assays of the binding of LTA to resealed right-side-out and inside-out membrane ghosts demonstrated that the outside surface was able to bind over 10 times more LTA than the inside surface. These results support the concept that the membranes possess specific binding sites for LTA and inciate that these binding sites are located almost entirely on the outside surface of erythrocyte membranes.