We examined the role of accessory cells that cooperated with the cloned cytolytic T cell line, L3. Accessory cells and T cell growth factors (TCGF) acted synergistically to stimulate cloned L3 CTL to grow and produce cytolytic activity. Accessory cells were non-T, radioresistant, plastic adherent M phi-like cells that interacted with cloned L3 CTL via soluble factors. Peritoneal M phi, at low density, also cooperated with L3 cells but at high density, inhibited L3 cells. Both accessory and inhibitory M phi acted upon L3 CTL without the intervention of other T lymphocytes or nonadherent cells. Other published reports indicate that M phi may promote cytolytic activity indirectly, amplifying TCGF production by helper T lymphocytes. This report indicates that M phi may also promote cytolytic activity directly, amplifying the response of CTL to TCGF.