Protein binding of chlorpromazine, propranolol, meperidine, desipramine, salicylic acid and phenytoin was determined in plasma of 64 healthy volunteers (35 males and 29 females). An attempt was made to identify factors affecting the plasma protein binding of these drugs. Whereas plasma albumin levels decreased as a function of age in both sexes, alpha 1-acid glycoprotein levels increased with age, but the increase was more pronounced in males. The free plasma fraction of the acidic drugs (salicylic acid, phenytoin) and desipramine (a base) showed a significant (p less than 0.005) negative correlation with plasma albumin levels. The free fractions of the other three basic drugs (chlorpromazine, propranolol, meperidine) in plasma showed a significant (p less than 0.005) negative correlation with alpha 1-acid glycoprotein plasma levels. Plasma protein binding of salicylic acid, phenytoin and desipramine decreased as a function of age. Plasma protein binding of chlorpromazine, propranolol and meperidine was virtually unaffected by age or was slightly increased (chlorpromazine). Only in the case of salicylic acid could a statistically significant difference be demonstrated between males and females in the free fraction-age relationship. Stepwise multiple linear regression analysis, including age and blood chemistry values such as hematocrit, bilirubin, cholesterol, triglycerides, creatinine, BUN, albumin and alpha 1-acid glycoprotein as independent variables, identified age as the variable explaining most of the variability in plasma binding of salicylic acid, phenytoin and desipramine. For chlorpromazine, propranolol and meperidine alpha 1-acid glycoprotein was the most important determinant of plasma protein binding.