The human plasma protein binding of disulfiram and its metabolite, diethylthiocarbamic acid methyl ester has been studied by an ultrafiltration technique. Both compounds were bound principally to albumin (mean 96.1 and 79.5%) over the ranges 200-800 and 345-2756 nM, respectively. The average number of binding sites was approximately 1 for both substances, whereas the average association constants were 7.1 x 10(4) and 6.1 x 10(3) M-1, respectively. Log P (octanol/water) was 2.81 for disulfiram and 1.85 for diethylthiocarbamic acid methyl ester.