Monoclonal antibody subplate-1 (mAb SP1) specifically stains somata, dendrites and axons of spiny inverted pyramidal neurons in the subplate zone in the early postnatal kitten neocortex. The SP1 antigen has been previously identified as a cytosolic protein of apparent molecular weight 56 kDa. We have now employed immune-affinity chromatography to further characterize this antigen. An antigen with SP1-like immunoreactivity (ir) is present in various organs, and is particularly enriched in blood plasma. Exsanguination of the organs prior to protein extraction reduces the SP1-ir band dramatically, indicative of a blood-borne molecule. The 56 kDa SP1-ir antigen was purified from plasma by affinity chromatography and subjected to Edman degradation. The first 20 N-terminal amino acids show 80% homology to the N-terminus of immunoglobulin heavy chain of man, the mouse and the dog. If the 56 kDa SP1-ir antigen in plasma is an immunoglobulin, and if an immunoglobulin-like molecule is present in the subplate, then antisera against cat immunoglobulins should stain subplate neurons. A polyclonal antiserum against cat IgG intensely stains the somata and dendrites of subplate neurons. On protein blots, this antiserum recognizes the 56 kDa band, and an additional band of approximately 27 kDa, corresponding in size to immunoglobulin light chains. Preabsorbing mAb SP1 with cat immunoglobulin G abolishes the immunoreactivity in sections of kitten cortex. Further, it dramatically reduces the reactivity on protein blots. The results suggest that the 56 kDa SP1-ir antigen in cortical subplate neurons belongs to the immunoglobulin superfamily.