The two-site enzyme immunoassasy (EIA) using the monoclonal antibody (MAb) 27/21 is a valuable method capable of detecting mouse and human NGF quantitatively (Soderstrom et al., 1990). The presence of NGF in serum has been controversial, since t6he previous assay methods failed to detect circulating NGF. Recently, we described the immunological detection of low levels of NGF in human serum samples and introduced a blocking test validating the specificity of the immunoreactivity for NGF in human serum (Lorigados et al., 1982). In the present work, we applied this two-site EIA using monoclonal NGF antibody 27/21 in the study of NGF serum levels from diverse neurodegenerative disorders. We also studied evolutive samples of Parkinson's patients that received neural transplant