The retention mechanism of zwitterion solutes in micellar liquid chromatography is discussed. Five cephalosporins, cephazolin, cephalexin, cephalothin, cephaloridine and cephradine, and two precursor acids of these antibiotics, 7-aminocephalosporanic acid and 7-aminodesacetoxycephalosporanic acid, were studied using aqueous sodium dodecyl sulfate as the mobile phase. The influence of variables such as the surfactant concentration, pH and the presence of several organic modifiers was also studied. The partition coefficients among the bulk solution and the micelles and the stationary phase were calculated.