Spleen cells from mice immunized with a cultured human neuroblastoma were hybridized with the mouse plasmacytoma P3X63Ag8. Hybrid myelomas were screened for production of antibodies that reacted with human neuroblastomas but not with cells from other tissues. One of these hybridoma antibodies reacted with an antigen present on the six human neuroblastomas tested, one of two retinoblastomas, a glioblastoma, and fetal brain, but did not react with other tumors or tissues including adult human brain.