Male Fischer rats were fed semipurified diets containing 0, 1, 100, and 1000 ppm benzo[a]pyrene (BaP) for 6 or 13 wk. Plasma samples were assayed for α‐fetoprotein (AFP) by a new sandwich‐type radioimmunoassay (RIA) utilizing a special controlled porous‐glass solid phase. This procedure is described in detail. Significant AFP elevation (p ≤ 0.01) was observed in the highest BaP treatment group after 5 wk of treatment. The 1 and 100 ppm BaP groups exhibited no AFP elevation throughout the study. Liver sections from the 1000 ppm groups had discrete γ‐glutamyl transpeptidase (GGT)‐positive foci 10–20 cells in diameter by the 6th wk. GGT‐positive foci were not evident in liver sections from the other treatment groups. Thus a high level of dietary BaP appears to rapidly alter rat liver cells, indicating hepatic neoplasia.