The polycyclic aromatic hydrocarbon (PAH)-induced expression of the rat Cyp1A1 gene is a complex process which appears to be regulated by several trans-acting factors including the 8S (Ah receptor) and 4S PAH binding proteins. This gene is closely associated with aryl hydrocarbon hydroxylase enzyme activity (AHH), which is known to bioactivate PAHs. The current study was undertaken to examine the hepatic 4S PAH binding protein in male Harlan Sprague-Dawley (HSD) rats using sucrose gradient analysis of 100,000 g supernatant solutions incubated with 10 nM [3H]benzo[a]pyrene (B[a]P) and a 200-fold excess of competitive ligands. Our results indicate that the 4S PAH binding protein is present in HSD rats and exhibits specific and saturable binding to B[a]P, but not to the dioxin congener 2,3,7,8-tetrachlorodibenzo-p-furan. The detection of the 4S protein in Harlan rats with B[a]P was found to be dependent on ligand and protein concentration. However, under standard assay conditions (10 nM [3H]B[a]P and 4.0 mg/ml cytosolic protein), HSD rats contained equivalent amounts of the 4S PAH binding protein compared to standard SD rats. Under standard conditions, no specific binding was observed to the 8S protein in HSD rats when B[a]P was used as the radioligand. Our data suggest that the role of the 4S protein in Cyp1A1 gene regulation in Harlan rats requires further evaluation.