The population growth kinetics of human diploid skin fibroblasts derived from cystic fibrosis homozygotes and heterozygotes and from normal subjects were investigated. Our data suggest the following: 1. Population doubling times increase with time in culture, with no significant differences observed among the three genotypes tested, when data were compared at the same subculture times or phases of growth. 2. The fraction of dividing cells in a population decreased with the duration in which the cells were in culture. No significant differences were obtained for cells derived from the three genotypes. 3. Cell cycle times were very similar (18-20 hr) when comparing the normal and cystic fibrosis lines or when comparing cystic fibrosis lines in phases 1 and 2 of growth. 4. No significant variations in population doubling times or growth fractions could be attributed to age or sex of the biopsy donor. 5. Variability in growth fractions and doubling times was minimal through the eighth subculture period but was very great in older cultures (tenth subculture). 6. Changes in growth fractions and doubling times appear to be due to the possibility of "aging" of human diploid fibroblasts in culture rather than to the presence or absence of genes for cystic fibrosis. 7. It is strongly indicated that differences in cell kinetic parameters cannot be used as the basis for differentiation between cells derived from normal or cystic fibrosis genotypes.