The present study was undertaken to examine the differential expression of asialo GM1 (AsGM1) on the responding cells and effectors of alloreactive cytotoxic T lymphocytes (CTL) and lymphokine-induced activated killers (LAK). It was found that AsGM1 was expressed on the 3-day-cultured LAK effectors. Its expression gradually disappeared to the extent that AsGM1 became undetectable after 5 to 6 days of culturing. In contrast, AsGM1 was detected on 3-day CTL generated in mixed-lymphocyte cultures (bulk cultures); however, the levels of AsGM1 expression remained the same for at least 7 days. When examining the expression of AsGM1 on the responding cells, the reciprocal results were obtained. AsGM1 was expressed the LAK responders, but we were unable to demonstrate AsGM1 on CTL responders. Depletion of AsGM1+ cells from the responding population reduced subsequent CTL responses; however, CTL responses could be restored by adding conditioned media containing both interleukin 2 (IL-2) and other helper-T-cell factors and could not be restored by purified IL-2 alone adding at comparable doses. Reconstituting the AsGM1-depleted responders with Lyt-2-depleted splenocytes also restored the CTL response. Furthermore, depletion of AsGM1 cells from the responding population did not reduce the precursor frequency of allo-CTL, whereas the precursor frequency of LAK cells was reduced 42-fold. These findings show that the reduction of CTL responses after depletion of AsGM1+ cells was not due to the removal of precursors; instead, the defect appeared to be in the helper population. We further found that the helper defect was not due to impaired IL-2 production, because the endogenous production of IL-2 AsGM1-depleted responders was not reduced. Therefore, AsGM1+ cells may play a role in the helper pathway other than IL-2 production.