Nerve growth factor was measured in cultured human skin fibroblasts from controls and from patients with familial dysautonomia and dystonia musculoram deformans. Cells from these sources grown over a range of cell densities contained similar levels of beta-nerve growth factor as measured by radioimmunoassay. Results of bioassay demonstrated that the nerve growth factor from dysautonomic cells was only approximately 10% as active per ng of immunoreactive protein as that from control and dystonic cells. Treatment of fibroblasts with the beta-adrenergic agonist isoproterenol resulted in a 17- to 170-fold rise in the cyclic AMP content of both control and dysautonomic cells in 10 min. The content of immunoreactive beta-nerve growth factor in control fibroblasts increased 50--300% after 3--4 hr of exposure to isoproterenol. At no time, throughout a 7.5 hr period, was there a change in the amount of immunoreactive beta-nerve growth factor in the dysautonomic cells. These studies suggest that the molecular basis of the genetic defect in familial dysautonomia may lie in processing of the precursor or in the structure of the biologically active beta subunit of nerve growth factor.