Nerve growth factor (NGF) is the best characterized of the neurotrophic factors, but there is incomplete information concerning its levels in body fluids. Normal values of NGF in serum from 157 normal subjects were determined by enzyme immunoassay (EIA). A mean NGF level of 194 +/- 25 pg ml-1 was obtained. There were no statistically significant variations with age, but the NGF level was significantly lower in females (112 +/- 31 pg ml-1) than in males (243 +/- 35 pg ml-1).