SummaryThirteen cystic fibrosis and 12 normal strains of skin fibroblasts obtained from the Institute for Medical Research were compared for their degree of production of cyclic adenosine 3′:5′-monophos-phate in response to isoproterenol and prostaglandin E1. There were no significant differences in their quantitative responses in content of cyclic AMP at two different times whether these cells were growing exponentially or were already confluent. All strains responded similarly to the presence of two types of phosphodiesterase inhibitor. The averaged initial rates of the response to isoproterenol in exponentially growing cells were also similar for the two sets of strains. Although response differed greatly between strains, the response of each strain was relatively reproducible.SpeculationIt appears that cystic fibrosis skin fibroblasts and normal skin Fibroblasts are equally sensitive to isoproterenol. The likely explanation for a previous observation of a 3-fold higher response to isoproterenol is that an improbable selection of the five cystic fibrosis strains and the five normal strains occurred.