SummaryA monoclonal antibody, MI/N1, is described that reacts predominantly with fresh neuroblastoma tissue, human neuroblastoma cell lines, and cells of the myeloid lineage. Investigation of the binding of this antibody to four different neuroblastoma cell lines showed CHP 100 bound approximately 4 times more antibody than CHP 126. Only 30% of the cells in the line CHP 100 bound MI/N1 as determined by indirect immunofluorescence. Thus, both quantitative and qualitative differences in the expression of antigen recognised by MI/N1 are detected on human neuroblastoma cell lines. Inasmuch as only five of eight marrow aspirates heavily infiltrated with neuroblasts bound the monoclonal, this also suggests a heterogeneity in antigenic expression on fresh tumour cells. Absorption studies indicate that the antigen recognised by MI/N1 is present on human foetal brain and adult human cerebellum. At a dilution of 1/750, equal volumes of foetal brain and adult cerebellum absorbed out 30 and 60% of the reactivity to the human neuroblastoma cell line CHP 100. No reactivity was found towards murine neuroblastoma cells or rat brain.Expression of antigen on cells in the myeloid lineage appears dependent upon their stage of maturation, increasing as cells mature to neutrophils and eosinophils.It is suggested that the quantitative and qualitative differences seen in the expression of antigen on neuroblastoma cells may relate to their being blocked at different stages of differentiation.