A link was proposed between human non‐Hodgkin's lymphoma and exposure to 2,4,5‐trichlorophenoxyacetic acid (245T) and pentachlorophenol (PCP). To test this view and the hypothesis that immune suppression or stimulation could affect B‐cell lymphoma (BCL) induction, we administered chronically to MRC‐Wistar (MRC‐W) rats of both sexes 98% pure 245T (600 mg/kg diet), 86% pure PCP (500 mg/kg diet), methyl‐prednisolone (20 mg/kg ip weekly), and Freund's adjuvant (0.5 ml im every 3–6 wk) for40 wk, together with 75 mg 2‐hydroxyethyInitrosourea (HENU)/1 drinking water, a system known to induce B‐cell lymphoma.

The 245T was shown to contain only 1–4 μg/kg each of 2,3,7,8‐tetrachloro‐dibenzodioxin (TCDD) and 2,3,7,8‐tetrachlorodibenzofuran (TCDF), but the PCP contained 25 μg TCDD and 670 μg TCDF/kg. HENU given alone induced B‐cell lymphoma and osteosarcoma as before, with higher incidences of both tumors in males than in females. The B‐cell lymphoma diagnosis was confirmed by immunologic typing of cell‐surface markers and by probes for gene rearrangements. Coadministration with HENU of three of the four test agents did not affect tumor incidence, but PCP acted synergistically with HENU to induce acute myelocytic leukemia. PCP given alone or with HENU induced a 40–67% incidence of liver cell adenomas in the female rats. These effects were probably not due to TCDD in the PCP. HENU induced acute myelocytic leukemia and lung tumors in Wistar rats and n‐butyInitrosourea induced acute myelocytic leukemia in MRC‐Wistar rats, indicating that B‐cell lymphoma induction was specific to the HENU‐MRC‐Wistar rat model.