The uptake of chloroquine (CQ) in blood cells in vitro is proportional to the concentration added to whole blood and follows the rank order of capacity: erythrocytes greater than platelets greater than granulocytes greater than lymphocytes. The accumulation of CQ in granulocytes and lymphocytes was linearly proportional to the concentration in whole blood. In average, the cellular compartment accumulated in vitro 90.6% of the total CQ amount in blood. The sum of CQ amount in isolated and washed blood cells was approximately 62.8%. The amount of CQ in erythrocytes was 4.05 to 8.4 times higher in comparison with platelet poor plasma. The number of platelets limits the uptake of CQ. With 1.5 x 10(-4) mol/l concentration in whole blood each platelet accumulated about 120 pg of CQ. The mechanism by which CQ accumulates in blood cells is not known. Possibly an active, but most probably a membrane perturbing, mechanism may play a role in CQ uptake. The compartmentation of CQ in blood must be taken into consideration when evaluating the disposition and pharmacokinetics of CQ in man.