The protein binding of GP53,633 [2-tert.butyl-4(5)-phenyl-5(4)-(3-pyridyl)-imidazole], a basic non-steroidal anti-inflammatory drug (NSAID), has been investigated. Although GP53,633 is a base (pKa 6.4) rather than an acid, the binding in plasma or serum was totally accounted for by binding to albumin. Scatchard analysis of the binding to albumin suggested the presence of one high affinity site and a number of low affinity sites. GP53,633 and its major metabolite, CGP8716, displaced site-I fluorescent probes (DNSA and warfarin) but not the site-II probe dansylsarcosine. Binding studies by equilibrium dialysis showed that GP53,633 and its metabolite displaced site-I drugs but not site-II drugs, and 14C-GP53,633 was itself displaced by site-I but not site-II drugs. As with other site-I drugs, the binding of GP53,633 was enhanced by addition of oleic acid at molar ratios of up to 2:1 with albumin. Albumin binding of GP53,633 was markedly increased by raising the pH from 6.0 to 8.5 suggesting that only the unionised drug can bind at site-I. The data are consistent with the major part of the binding energy at site-I being due to hydrophobic interactions and also suggest that there is a cationic centre on the protein at or near site-I which precludes the binding of positively charged drugs.