The amount of chloroquine and desethyl-chloroquine was determined in samples of total blood and in blood cell fractions from three normal subjects after one oral dose of 1000 mg of chloroquine diphosphate. About 70-85% of the total whole blood content of chloroquine and its metabolite desethyl-chloroquine were recovered in blood cells isolated from whole blood, indicating that these compounds have a high cell/plasma concentration ratio. They were mainly present in thrombocytes and granulocytes. A study of 40 patients taking chloroquine regularly as a treatment for rheumatoid arthritis showed significantly higher concentrations of chloroquine and desethyl-chloroquine in serum than in plasma. The concentration of chloroquine was about two times higher in serum than in plasma and for desethyl-chloroquine the concentration was about four times higher in serum than in plasma. These differences may be explained by a release of chloroquine and desethyl-chloroquine from thrombocytes during the coagulation of blood. The practical implication of the results is that the samples taken for chloroquine determination must be clearly identified as serum or plasma.