The MDCK cell line has been used as a model for the renal distal tubule. When grown on permeable supports, the cells form tight junctions and transport sodium and calcium. Widely different values have, however, been obtained for resistance and transports by different investigators. Since the MDCK cell line is genetically heterogeneous, one potential reason for observed differences is the use of different populations of cells. Dilutional techniques have been used to obtain eleven MDCK cell clones. The clones exhibit a range of values for resistance (300-4000 ohms cm2), net Ca2+ transport (3.2-81 pmol/cm2/min) and net Na+ transport (0.4-6.0 nmol/cm2/min). To determine if the characteristics of clonal cell lines changed with increasing passage number, resistance was measured at relatively early and late passage number. Resistance increased with increasing passage number in two of six lines and decreased in one. These studies demonstrate that differences between previous studies may be due to differences in cell population, length of time since passage, and passage number. Clonal cell lines should provide a useful tool to investigate ion transport and mechanisms of regulation of transport in this model of renal distal tubule.