Aberrant biological characteristics of established cell lines and clones, derived from nitrosourea-induced gliomas in CDF rats were compared with normal rat glial cells. Despite their tumorigenicity and unrestrained proliferation due to inherent cytogenetic anomalies, these neoplastic cells retained certain normal glial attributes to a variable degree. Differentiated glioma cells resembled normal glial cells to a greater extent than they resembled anaplastic glioma cells. They were also less malignant upon implantation. Cell cycle analyses revealed that considerable variations not only for the G1 phase but also for the S period were demonstrated among different tumor cell types. Shortening of the G1 phase may dictate a shorter generation time (shorter doubling time) since a larger potential proliferative pool may overcome the effect of a prolonged generation period may constitute a faster proliferation rate. Although the cell generation period of neoplastic cells is not necessarily shorter than that of normal glial cells, the lower proportion of nonproliferative cells results in a much faster growth rate when compared wo non-neoplastic glial cells in culture.