MK-927 [5,6-dihydro-4H-4(isobutylamino)thieno(2,3-B)thiopyran -2-sulfonamide-7.7 dioxide], a potent carbonic anhydrase inhibitor, contains a chiral center and exists as a racemate. In order to understand the kinetic behavior of the enantiomers of MK-927 in the body, the uptake and binding of these compounds were studied in human erythrocytes in vitro. Since no degradation or metabolism of the enantiomers occurred during incubation in blood, one can describe the equilibration of the drugs between plasma and erythrocytes by a closed two-compartment system. Erythrocytes were considered as a compartment composed of two parts: one in which free drug is exchangeable to plasma and the other in which drug is tightly bound to carbonic anhydrase in a Michaelis-Menten type binding. After the addition of the enantiomers individually to fresh blood, they were taken up by erythrocytes rapidly in a concentration-dependent manner. The time to achieve equilibrium decreased as the concentration increased, suggesting saturation of binding sites. With the assumption of simple diffusion, the binding and transfer kinetics were determined simultaneously by computer fitting. There were no stereoselective differences in the transfer process of the enantiomers across the erythrocyte membrane, while binding of the enantiomers exhibited stereoselectivity. The penetration of the unbound enantiomer across the erythrocyte cell membrane was rapid, with a mean transit time of about 3 sec. The S-(+)-enantiomer was bound to the high-affinity carbonic anhydrase isoenzyme more strongly than the R-(-)-enantiomer by approximately 10-fold. For the low-affinity isoenzyme, the R-(-)-enantiomer was bound more strongly than the S-(+)-enantiomer.