The authors evaluated the contribution of the SLCO2B1 polymorphism to the pharmacokinetics of celiprolol at a microdose (MD) and therapeutic dose (TD) and compared pharmacokinetic proportionality between the 2 dose forms in 30 SLCO2B1 genotype-matched healthy volunteers. Three drugs (celiprolol, fexofenadine, and atenolol) were orally administered as a cassette dosing following the MD (totally 97.5 µg) and then a TD (100 mg) of celiprolol, with and without grapefruit juice. The mean AUC(0-24) of celiprolol was lower in SLCO2B1*3/*3 individuals (775 ng·h/mL) than in *1/*3 (1097 ng·h/mL) and *1/*1 (1547 ng·h/mL) individuals following the TD, and this was confirmed in population pharmacokinetic analysis with statistical significances; however, SLCO2B1 genotype-dependent differences disappeared following the MD. Dose-normalized AUC of celiprolol at the MD was much lower than that at the TD, explained by the saturation of the efflux transporter. Thus, the effect of SLCO2B1 polymorphism on the AUC of celiprolol clearly observed only at the TD may be due to the saturation of the efflux transport systems.