The electrophoretic mobility of human diploid fibroblasts, TIG-1, was studied at different passages. The net negative surface charge of the cells decreased from -1.658 +/- 0.108 micron/s/V/cm at an early passage (15 population doublings, PD) to -1.173 +/- 0.116 at the final passage (67 PD) in 1/15 M phosphate buffer supplemented with 5.4% glucose. The decrease was slow at 15-45 PD, but was rapid at 45-67 PD. The net negative surface charge of small cells in the late passage populations was not different from that of larger cells in this population, and was significantly lower than that of small cells in the middle passage populations. The distribution of the mobilities of cells in each passage was independent of the size of the individual cells, and the mean value was distinct for the passage number. The viability of the cells was retained during the assay of electrophoretic mobility under these conditions. These results indicate that the net negative surface charge of human diploid fibroblasts represents a cell surface maker for in vitro cellular age in the population.