The interaction of L‐(1‐3H) methadone with solutions of purified human gamma globulin and with human plasma was studied by equilibrium dialysis. The per cent of methadone bound to gamma globulin was dependent on drug and protein concentration. In the normal range of serum gamma globulin concentration and the therapeutic range of plasma methadone concentration, gamma globulin bound 13.4% to 17.4% ot the total drug concentration. The binding of methadone to plasma from 2 healthy male volunteers, nontolerant to narcotic analgesics, was dependent on drug concentration. Methadone bound to plasma proteins decreased from 87.3% to 83.7% as drug concentration increased in the therapeutic range. This small decrease is most likely due to decreased binding by the gamma globulin fraction since albumin binding is constant in this range. The data in this study, as weil as a previous study with albumin, suggest that methadone may also bind to other plasma proteins such as beta, alpha‐I, and alpha‐2 globulins.