The in vitro longevity of skin fibroblasts in several parallel cultures from each of 4 cystic fibrosis (CF) patients and 3 control patients was measured by serial passages. The control cultures reached 51–56 passages before growth ceased. 1 CF strain achieved an average of 69 passages and another 64 passages. Both of these longevities are significantly greater than the controls. 2 CF strains had significantly shorter longevities than the controls. The results indicate that the CF gene is expressed in cultured fibroblasts and provide further evidence that there may be more than one form of the diseas