Spleen T cells from phosphorylcholine (PC)-primed BALB/c mice were indirectly stained with affinity column-purified heterologous rabbit anti-TEPC 15 (T 15) antibodies. These antibodies were shown to be directed towards antigenic determinants in or near the combining site of T 15 in that their interaction with the myeloma protein was more than 95% inhibitable by PC. The stained cells were sorted into "bright" and "dull" fractions using a fluorescence-activated cell sorter. Sorted populations were then assessed for enrichment and depletion of PC-specific helper activity using an in vivo adoptive transfer assay. The results indicate that the staining procedure employed enabled the specific enrichment of PC helper T cells and therefore suggest that these T cells bear combining site structures similar to that of the myeloma protein T 15.