Monoclonal antibody (MAb) UJ13A, raised against 16-week-old human foetal brain, recognizes an antigen present on the majority of tissues of neuro-ectodermal origin. The binding profile of this antibody is similar to the known distribution of the neural cell adhesion molecule (NCAM). Although detailed immunohistological and immuno-cytochemical studies with the MAb have been published previously, we demonstrate here, through investigations of selected tissues and cell lines, that the binding profile of an anti-NCAM antiserum is similar to UJ13A. Additional indirect evidence suggesting that UJ13A recognizes NCAM comes from Northern blot analysis of cell lines either binding or not binding UJ13A as determined by indirect immunofluorescence. Only those cell lines known to bind UJ13A express high levels of NCAM mRNA. Western blot analysis of extracts of human brain show that UJ13A recognizes proteins of 180 and 140 kDa in addition to a very weak band of 120 kDa. This data corroborates the suggestion that UJ13A recognizes NCAM as these 3 isoforms of the protein are identified in human brain. Final confirmation of the specificity of UJ13A comes from the study of 3T3 fibroblasts transfected with a cDNA coding for the 125 kDa isoform of human muscle NCAM. UJ13A selectively recognizes these transfectants and binds to a 125 kDa protein isolated from the cells by Western blot analysis. Thus we conclude from these immunological, biochemical and molecular studies that the antigen recognized by the UJ13A MAb is the neural cell adhesion molecule.