The activities of the retinoid, N-(4-hydroxyphenyl)-all-trans-retinamide (4-HPR) and the polyamine synthesis inhibitor, alpha-difluoromethylornithine (DFMO), as inhibitors of lymphoma induction in PIM transgenic mice were evaluated. Lymphoma was induced in male PIM mice by a single intraperitoneal injection of 50 mg N-ethyl-N-nitrosourea (ENU) per kg body weight. Continuous dietary administration of 4-HPR (391, 196 or 98 mg/kg diet) or DFMO (1000, 500 or 250 mg/kg diet) was initiated immediately after ENU administration, and was continued until the end of the study at 35 weeks. At 20 weeks post-ENU, the high dose of 4-HPR reduced both lymphoma incidence and associated mortality. However, the protection conferred by 4-HPR represented a delay rather than an inhibition of neoplastic development, since both lymphoma incidence and mortality at study termination were similar in dietary controls and all groups treated with 4-HPR. DFMO had no effect on lymphoma incidence, latency or mortality at any point in the study. These results suggest that 4-HPR or other retinoids may be effective in the prevention of lymphoma induction, whereas inhibition of polyamine biosynthesis does not appear to present a useful mechanistic target for the chemoprevention of lymphoid neoplasia. The PIM transgenic mouse provides a useful in vivo model for the rapid evaluation of chemopreventive agents.