Ten patients with chronic occlusive arteriosclerosis received single oral doses of 100, 200, 400, 800, and 1200 mg pentoxifylline in a single-blind, placebo-controlled study. Blood samples were drawn at baseline and at 2 hour intervals for 6 hours. Drug and metabolite levels, as well as red cell filterability (deformability), were determined on all blood samples. Statistically significant dose-response increases of red cell filterability were found 4 and 6 hours after oral medication with the dosages of 200 to 1200 mg pentoxifylline. These changes were proportional to the plasma levels of pentoxifylline and metabolites 1 and 5 of this agent. Attempts were made to develop a suitable animal-screening method for agents with similar activity and to determine whether red blood cells in the absence of disease-related abnormalities may respond to this type of therapy. Five healthy Macaca arctoides monkeys were given 24 mg/kg pentoxifylline intravenously, and measurable but lesser increases in red cell deformability were recorded than in the patients.