A human malignant glioma cell line, LN-18, has been established in monolayer culture and subcultured for more than 115 passages. LN-18 cells grow in vitro as bipolar or stellate cells with pleomorphic nuclei, have a doubling time of about 72 h and a plating efficiency of 3%. The glial nature of these cells has been assessed by ultrastructural examination. The synthesis of glial fibrillary acidic and S-100 proteins could not be demonstrated, although the initial biopsy tissue and the early cultures were positive for the former. The presence of Ia-like antigens on the surface of these cells was demonstrated using allo and xeno antisera. LN-18 cells were also shown to synthesize large quantities of fibronectin. The injection of LN-18 cells into nude mice induced the formation of solid tumor masses that could be retransplanted every 3 weeks and showed a morphology comparable to that of the initial biopsy. Karyotype analysis revealed the presence of three marker chromosomes, constantly present before and after hetero-transplantation.