The binding properties of codeine, morphine (as representative opium alkaloids), and methadone (a synthetic pharmacologically similar compound) were studied with selected human serum proteins. The methodology involved equilibrium and dynamic dialysis using 3H-and/or 14C-labeled compounds. For estimation of the percent binding with equilibrium dialysis, concentrations of the ligand used were approximately therapeutic blood levels and another concentration 30-60 times higher. The percent binding to whole human serum ranged from about 20% for morphine to almost 60% for methadone. Of the human serum proteins investigated, the highest percent binding was found with albumin, except for methadone for which it was beta-globulin III. The affinity for other serum proteins varied with the ligand. In studies with albumin using dynamic dialysis, the plots of nubar divided by free concentration versus nubar were similar for all three ligands studied and had positive slopes, unlike those reported for acidic compounds for which the slope is always negative. In studies of binding of one ligand in the presence of another, significant competition was demonstrated, suggesting that the same binding sites were involved.