Cultures of rat astroglial (AG) cells treated with the ganglioside GM1 in serum-free medium respond with an increase in DNA labeling and cell proliferation. However, GM1 doses above 60 microM cause decreasing DNA labeling to levels even below that determined in the absence of added GM1. Quantitative determination of cell numbers in 24-hr cultures treated with various GM1 concentrations shows no loss of cells but a progressive shift in cell morphology from the usual flat to a rounded shape, suggesting a GM1-induced progressive reduction in cell-substratum adhesion. The rounded cells, which can be readily washed off the culture wells, do not carry out measurable DNA synthesis but do appear normal by several other biochemical measurements. All the GM1-treated rounded cells can regain their flat morphology and resume DNA synthesis and cell replication if fetal calf serum is added to the medium. We conclude that modulation of cell adhesion is a major component of astroglial cell responses to exogenous ganglioside treatment.