The transport characteristics of cefadroxil, an aminocephalosporin antibiotic, across the brush border membrane of rat small intestine were investigated by a rapid filtration technique. The uptake of cefadroxil was not affected by Na+ gradient, suggesting the absence of a cotransport system between cefadroxil and Na+ in the brush border membrane. The uptake was slightly inhibited by HgCl2 pretreatment and stimulated by the countertransport effect, where cyclacillin played a role as an elicitor. These results suggest the existence of a carrier-mediated transport system for cefadroxil in the brush border membrane, which is shared with cyclacillin. Papain treatment increased the specific transport activities for the antibiotic. This may be the first step of purification of the cefadroxil transport carrier.