Knowledge of drug protein-binding and blood cell partitioning may be important for evaluating the pharmacokinetic parameters of zidovudine, particularly because of its intracellular site of action and potential to induce side effects. Equilibrium dialysis studies of zidovudine were performed over 2 h to identify the extent and site of binding. Zidovudine was added to anticoagulated whole blood to study blood cell distribution over a 24 h period at 37 degrees C and at 21 degrees C. Concurrent plasma and whole blood samples were determined at various time-points and blood partitioning was determined by application of a mass balance equation. All samples were analyzed using radioimmunoassay. The free fraction of zidovudine at a concentration of 500 ng/ml (1.7 microM) was 0.77 +/- 0.05 in plasma, 0.78 +/- 0.03 in serum, 0.88 +/- 0.03 in 4 g/dl albumin solution, and 1.0 in 100 mg/dl alpha 1-acid glycoprotein solution. A free fraction of 0.72 +/- 0.10 was observed in plasma from HIV-infected patients with zidovudine concentrations ranging from 16 to 91 ng/ml. Zidovudine equilibration between plasma and blood cells occurred rapidly, being complete within 10 min. After equilibrium was complete, the mean whole blood:plasma ratio was 0.86 +/- 0.02 and 0.80 +/- 0.04 (P = 0.20) and mean blood cell Partitioning ratio, [cell]/[plasma-free], was 0.85 +/- 0.06 and 0.66 +/- 0.14 (P = 0.25) for studies at 37 degrees C and 21 degrees C, respectively. The partitioning ratio was relatively consistent over the study period, suggesting no accumulation in blood cells. These results suggest that zidovudine binds to a small extent primarily to albumin. The free concentration equilibrates readily between blood cells and plasma independent of concentration and without signs of accumulation.