The partitioning of 3H-thioridazine and 3H-mesoridazine in fresh human whole blood was studied. The packed red blood cells were solubilized using the New England Nuclear protocol for whole blood solubilization. The plasma fraction was further fractionated into protein bound and free drug by molecular ultrafiltration. All solutions were counted in Biofluor LSC cocktail and corrected for quenching. Greater than 99% of the labeled drug was bound to the red blood cells and plasma protein. For thioridazine, 59% is bound to RBC, 41% is bound to plasma protein and 0.7% is free; for mesoridazine, 63% is bound to RBC, 37% is bound to plasma protein and 0.9% is free. Though substantial overlap is found in the bound percentage for mesoridazine and thioridazine, more mesoridazine binds to RBC than thioridazine (p less than 0.01). There is no statistically significant relationship between the amount of drug bound to the RBC or to plasma protein and the percent free drug. Though the total drug concentration is the same (1 microgram/ml) the percent free drug is quite variable across subjects by as much as a factor of three. Since free drug is the pharmacologically active portion and therefore determinant of clinical response, the reported variation in free drug concentration at the same total blood concentration invalidates the measurements of total serum or plasma drug concentration as predictive of clinical response.