Adhesion of leukocytes to the aortic endothelium was studied in specific pathogen-free (SPF) and conventional rats and in SPF rats with diet-induced hypercholesterolemia. Nonspecific esterase activity with alpha-naphthyl acetate as substrate was used to characterize the adhered cells. Phagocytic activity was determined by injecting i.v. 0.1-0.4 ml/100 g doses of Monastral blue B (MbB). Adhesion in SPF rats was 8 +/- 4 esterase (+) cells/mm2. Adhesion in conventional rats was of the same order except in 2 cases with antibodies to Mycoplasma pulmonis and Kilham rat virus, where adhesion was 44 and 68 esterase (+) cells/mm2, respectively. For all MbB doses studied, phagocytic activity arose in a percentage of the adherent cells, ranging from 5 to 85%. Rats fed the hyperlipidic diet for 15 days developed severe hypercholesterolemia and adhesion was drastically increased to 200-700 esterase (+) cells/mm2. Results indicate that: (1) spontaneous pathology in rats may produce an increased adhesion of leukocytes to the endothelium, and (2) phagocytic activity is only expressed in a fraction of the esterase (+) cells adhered to the endothelium.