The free (unbound) drug in serum is an important determinant of pharmacologic response. The present study was performed to more completely identify and evaluate factors affecting cocaine binding to human serum proteins. Protein binding was determined by ultrafiltration with [3H] cocaine. Cocaine binding parameters in serum from eight healthy volunteers were determined over a concentration range of 0.003 to 300 microM (0.001-100 micrograms/ml) and indicated cocaine binds to two classes of independent binding sites; one with high affinity [association constant (Ka) = 0.42 +/- 0.09 microM-1] and low capacity (N1 = 12.3 +/- 2.9 microM) and one with low affinity and high capacity (gamma = 0.41 +/- 0.05). Binding was concentration dependent with free fraction increasing from 0.16 +/- 0.05 to 0.68 +/- 0.02 over this concentration range. The binding capacity was significantly correlated with alpha-1-acid glycoprotein (AAG) concentration (r2 = 0.71, P = .0009). Binding studies were performed using AAG and human serum albumin (HSA) alone and together in phosphate buffer to determine the specific proteins responsible for cocaine binding. These studies revealed the binding of cocaine to AAG is potentiated by the presence of HSA as Ka for the first binding site increased from 0.08 microM-1 with AAG alone to 0.46 microM-1 with AAG combined with HSA 4 g/dl. Binding parameter estimates and cocaine free fraction in human serum and AAG 75 mg/dl plus HSA 4 g/dl in phosphate buffer were similar indicating that AAG and HSA are the principal binding proteins in serum.(ABSTRACT TRUNCATED AT 250 WORDS)