To examine the role of lymphocyte function-associated antigen 1 (LFA-1) expression on murine B cells as it pertains to their function in T cell activation, we carried out antigen-presentation assays in tissue culture wells coated with a purified, secreted form of the murine intercellular adhesion molecule 1 (ICAM-1). We observed a significant decrease in the concentration of antigen required to activate a T cell hybridoma and primary T cells in wells coated with ICAM-1. This effect was dependent on the amount of ICAM-1 used to coat the wells and was also observed in wells coated with anti-LFA-1-monoclonal antibodies and was blocked by soluble anti-LFA-1 antibodies. The effect on antigen dose was most pronounced in assays carried out with an ICAM-1-deficient mutant B lymphoma cell line, small resting primary B cells, and unfractionated primary B cells at low concentrations. No decrease in the antigen dose was observed if the B cells were chemically fixed or treated with ricin, or when antigen was presented by a HeLa cell line transfected with murine class II major histocompatibility complex (MHC) genes, indicating that the immobilized ICAM-1 was mediating its effect through B cell LFA-1, and that B cell protein synthesis was required. The enhancing effect was also observed if the B cells were prepulsed with antigen, indicating that improved uptake or processing of antigen, or increased class II MHC expression were unlikely mechanisms.