We investigated the characteristics of binding of tamsulosin to alpha(1)-acid glycoprotein (AGP) genetic variants. The binding of tamsulosin to each of the human AGP variants was determined by ultrafiltration, and the binding characteristics for each variant were compared using binding parameters and inhibition of the binding by disopyramide and warfarin. The affinities of tamsulosin binding to a F1/S variant mixture and total AGP variants were relatively high (dissociation constants 1.6 microM). On the other hand, the dissociation constant for variant A was 14.9+/-2.53 microM. The binding of tamsulosin was competitively inhibited by warfarin but not by disopyramide. Tamsulosin appears to be a suitable compound for studying the characteristics of drug binding to human AGP F1/S variants under clinical conditions.