Mouse monoclonal antibodies have been raised to the C-terminal region of rat type II voltage-dependent sodium channel. One of these, designated 13A4-G4, recognizes a 260 Kd putative sodium channel protein in human fetal brain, heart and skeletal muscle, at 14-18 weeks of gestation. Faint immunoreactivity is also present in human fetal kidney but none has been detected in human fetal liver, lung or spleen. The antibody reacts in both western blots and immunocytochemical preparations with the human neuroblastoma cell lines SK-N-SH and SK-N-MC, the rhabdomyoblastoma cell line TE671, Y79 retinoblastoma cells and IPSB-18 astrocytoma cells. No 13A4-G4 immunoreactivity has been detected in several human cell lines derived from tissues that do not normally express sodium channels.