The disposition of [14C]D-penicillamine (PA) was investigated in vitro and in vivo with male Wistar rats. Irreversible binding of [14C]PA to isolated rat plasma proteins in vitro reached a maximum of 20.6% of total radioactivity at 6 hr. Irreversibly bound [14C]PA could be dissociated with dithiothreitol, demonstrating that conjugation was via disulphide linkage. Three hours after i.v. administration of [14C]PA (27 mumol/kg) to rats 100% of plasma radioactivity was irreversibly bound, representing approximately 3.5% of the dose. Further studies on the disposition of PA-plasma protein conjugates showed that dissociation occurred readily in vivo: the plasma half-life of the conjugate was approximately 3 hr. Free [14C]PA was the major urinary metabolite after administration of both free and conjugated drug. These studies show that the disposition of PA is similar to that reported for the structurally related sulphydryl drug captopril (CP). Free PA (340 mumol/kg and 3.4 mmol/kg) administered i.p. and i.m. daily for 4 days at one monthly intervals, and also PA-KLH conjugate (100 micrograms/rat) administered by single i.p. injection at monthly intervals with and without Freund's complete adjuvant, failed to induce PA-specific IgG or IgM antibody responses detectable by ELISA. In contrast, CP (270 mumol/kg), administered by the same protocol as free PA, induced a CP-specific IgG antibody response after the third series of monthly injections. These data suggest that the difference in immunogenicity between PA and CP arises from a difference in the intrinsic immunogenicity of the haptens, rather than from their disposition.