A leukemia cell transplant model and both in situ and in vitro bioassays were used to assess the roles of endogenous factors in mediating diet restriction (DR)-induced inhibition of mononuclear cell leukemia (MNCL) in Fischer 344 rats. DR-treated male rats (n = 35), which were fed 75% of ad libitum (AL) intake of NIH-07 open formula diet, had lower transplanted MNCL incidence (54 versus 77%; P = 0.039) with longer latency (P = 0.015) and decreased severity (P = 0.01) than AL-treated rats 12 weeks after inoculation with MNCL cells. Five-day proliferation rates of cultured MNCL (CRNK-16) cells in diffusion chambers implanted in DR-treated rats were 22% less than in AL-treated rats (P = 0.03), indicating that DR-dependent diffusible factor(s) modulate in situ MNCL cell growth. Serum from DR-treated rats supported lower in vitro CRNK-16 cell proliferation rates relative to serum from AL-treated rats. Serum levels of both growth hormone (GH) and insulin-like growth factor 1 (IGF-1) were over 50% lower in DR- versus AL-treated rats. An evaluation of the in vitro cell proliferative activity of a panel of purified factors showed that GH and IGF-1, but not 15 other growth factors, stimulated thymidine incorporation in CRNK-16 cells. Infusion of either GH or IGF-1 via osmotic minipumps restored in situ and in vitro CRNK-16 cell proliferation in DR-treated rats up to rates measured in AL-treated rats. Splenocytes from DR-treated rats, relative to AL-treated rats, were more sensitive to mitogen stimulation, displayed increased cell surface expression of receptors for class 1 and 2 major histocompatibility complex molecules, and were more cytotoxic to target tumor cells. Infusion of either GH or IGF-1 in DR-treated rats further enhanced mitogen responsiveness and natural cytotoxicity but reversed the DR-induced increase in major histocompatibility complex receptors. We conclude that DR modulates MNCL progression in Fischer 344 rats through both its influence on MNCL cell proliferation via suppression of the GH:IGF-1 axis and its enhancement of host defenses against tumor cells.