Evidence has indicated that two soluble factors are essential for a T-cell proliferative response to antigen or lectin^1–3. One such factor, thought to be of T-cell origin, is distinguished by its ability to initiate and maintain continuous T-cell growth (T-cell growth factor, TCGF)^4–6. A second activity of macrophage origin augments lectin-initiated proliferative responses of thy-mocytes or adherent cell-depleted splenocytes (lymphocyte activating factor, LAF)⁷. Although it has been suggested that LAF promotes T-cell proliferation by facilitating the production of TCGF by T-cells^1–3, direct evidence supporting this hypothesis has been lacking. We describe here experiments with a murine thymic lymphoma which releases TCGF in response to lectin. In serum-free, defined medium, highly purified LAF promotes a concentration-dependent increase in TCGF release. Because the proliferation of T cells depends on the concentration of TCGF available^8,9, these results indicate that LAF and TCGF constitute an essential bimodal amplification system which ultimately determines the extent of T-cell clonal expansion.