The binding of cocaine (COC) and cocaethylene (CE) in human serum was studied by equilibrium dialysis. Scatchard analysis suggested a high-affinity binder (Ka, 2.56 x 10(4)L/mol; Bo, 7.38 x 10(-5) mol/L) and a low-affinity binder (Ka, 4.47 x 10(3)L/mol; Bo, 2.77 x 10(-4) mol/L) for COC. Two high-affinity binders (Ka, 5.21 x 10(4) L/mol; Bo, 2.54 x 10(-5) mol/L; and Ka, 4.32 x 10(4) L/mol; Bo, 2.43 x 10(-5) mol/L) were discernible for CE. For both compounds additional, very-low-affinity, high-capacity (nonspecific) binding was also seen. Supplementation of serum with specific proteins suggested that the high-affinity binding was due to alpha-1-acid glycoprotein, whereas the low-affinity binding was due to albumin, inasmuch as such supplementation increased the ratio of bound to free drug for both COC and CE.