Highly purified, detergent-solubilized HLA-A and -B antigens and HLA-D antigens were separately incorporated into liposomes. Detergent-solubilized transplantation antigens, but not papain-solubilized antigens lacking the membrane-integrated portions of the molecules, were bound to the liposomes. A considerable portion of the liposome-bound antigens displayed accessible antigenic sites, suggesting that they were oriented in the right-side-out direction. Liposomes containing the HLA-A and -B antigens or the HLA-D antigen interacted similarly with bacteria. The two types of liposomes bound efficiently to two strains of Neisseria catarrhalis (now classified as Branhamella catarrhalis) and to one strain of Haemophilus influenzae, weakly to one strain of Escherichia coli, and not at all to another strain of E. coli. The binding between the HLA antigen-containing liposomes and one strain of N. catarrhalis was abolished when Fab fragments directed against the heavy chains of HLA-A and -B antigens or against HLA-D antigens, respectively, were added. In contrast Fab fragments against beta(2)-microglobulin did not measurably impede the bacteria-liposome interaction, suggesting that, with regard to the HLA-A and -B antigens, the heavy, but not the light, chains interacted with the bacteria. Additional experiments showed that N. catarrhalis preferentially interacted with transplantation antigens when mixed with detergent-solubilized lymphocyte membrane glycoproteins. These data suggest that HLA-A and -B and HLA-D antigens may have the function of interacting with foreign antigens such as bacteria.