Thymus-derived lymphocytes (T cells) and bone marrow-derived lymphocytes (B cells) from mouse spleens bind specifically to antigen-derivatized nylon fibers. The fiber-bound population consisted of about 60%-70% B cells and 30% T cells as determined by cytotoxicity, fluorescence, and antibody-complement binding assays. Essentially all fiber-bound cells were viable and could be accounted for as T or B cells. Enriched populations of T or B cells could be isolated on the fibers by destruction of one or the other cell type with the appropriate antiserum plus complement. T or B cells could also be fractionated according to their relative affinity (or avidity) for a given antigen.