AbstractConsiderable controversy surrounds the question of whether or not nerve growth factor (NGF) or a related nerve growth‐promoting factor is present in serum. Recently, supporting its role as a local neuronotrophic factor, the presence of NGF in glial cells and its production in target tissues of NGF‐responsive neurons were demonstrated [Rush:Nature312:364–367, 1984; Heumann, Korsching, Scott, Thoenen:EMBOJ3:3183–3189, 1984; Shelton and Reichardt:Proc Natl Acad Sci USA81:7952–7955, 1984]. At the same time, the concept that NGF may play a role as a humoral factor has been questioned, since careful analyses of serum with specific and sensitive radioimmunoassays [Suda, Barde, Thoenen:Proc Natl Acad Sci USA75:4042–4046, 1978; Korsching and Thoenen;Proc Natl Acad Sci USA80:3513–3516, 1983; Furukawa, Kamo, Furukawa, Akazawa, Satoyoshi, Itoh, Hayashi:J Neurochem40:734–744, 1983] as well as bioassays [Skaper and Varon:Exp Neurol76:655–665, 1982] have not confirmed earlier reports [Levi‐Montacini and Booker;Proc Natl Acad Sci USA46:373–391, 1960; Banks, Banthorpe, Charlwood, Pearce, Vernon:Nature246:503–504, 1973; Hendry:Biochem J128:1265–1272, 1972] on NGF's representation in serum.In this study serum from mouse, rat, and man was analyzed with an in vitro bioassay system which employs sensory neurons from chicken embyro dorsal root ganglia and which allows the measurement of NGF concentrations as low as 0.8 pM. It was found that sera from all these species contained neuronotrophic activity (S‐NGF). The target cell spectrum as well as characteristic parameters of the neuronal growth response of S‐NGF and of NGF were identical. S‐NGF of mouse serum was completely inhibitable by polyclonal and monoclonal antibodies to mouse submandibular gland β NGF. On polyacrylamide isoelectric focussing gels mouse and human S‐NGF could be recovered from the same position as NGF as well as from the region where α2‐macroglobulin and serum albumin focused.In newborn and adult male and female mice basal S‐NGF levels were equivalent to 10–50 pM NGF. A. fraction of the serum samples of male mice showed elevated S‐NGF levels. The incidence of high S‐NGF levels was more frequent in NMRI and C57BL/6 males than in BALB/c males. Following sialectomy of male mice only basal S‐NGF levels were observed up to 5 weeks after the operation. This indicates that although the submandibular gland may contribute to S‐NGF levels in serum under certain conditions that appeared to be stre