Neuroblastoma is one of the most common solid tumors of childhood and is notable for its ability to spontaneously regress and, in some instances, to differentiate to less malignant ganglioneuromas. Since immune mechanisms may account for these phenomena, identification of in vivo immune responses to tumor cell surface antigens may be important to the progression of the disease. As determined by analysis on the fluorescence-activated cell sorter, sera from 10 of 18 neuroblastomas patients were found to contain antibodies to a cell surface antigen present on subpopulations of cells from human neuroblastoma cell lines maintained in vitro. Eight human neuroblastoma cell lines were examined and found to vary in reactivity with sera. Induction of differentiation of cell lines with retinoic acid (RA) in vitro resulted in most cell lines bearing higher percentages of positive cells but with a decreased mean cell fluorescence. Preliminary Western blot analysis of lysates of the human cell lines NMB/N7, SMS-KAN, and SK-N-MC showed two principal antigen bands on reducing gels. Comparison of sera from different individuals on lysates of cell lines showed reactivity principally with bands of 105-110 kD and 65-70 kD and an additional minor band of slightly lower molecular weight with the higher titer sera. The ability of different sera to recognize a common antigen pattern suggests that this represents an immunodominant cell surface antigen. Examination of reactivity of other cell lines in this system showed that positive sera reacted with all neuroblastoma lines examined, one neuroepithelioma (SK-N-MC), two melanoma lines (MeWo, G361), and one adrenal-derived adenocarcinoma (SW-13).