Human glial cultures of any passage consist of two populations of cells: those with mitotic ability, and the non-dividers. The fraction of non-dividers increases with age of the culture, and dominates in late passages. When cells from midphase II cultures (passage 28) were sparsely seeded in dishes containing agarose partially covered by small, isolated, palladium squares (haptotactic islands) they settled on the palladium squares but not on the agarose. 58% of the cells divided, and formed mini-clones which became density growth inhibited within 10 days in medium with 5% serum. The non-dividers comprised 42%. They showed a characteristic indolent motility pattern. When these cultures were exposed to 15% fetal calf serum and 2 ng/ml mEGF (mouse epidermal growth factor) for another 5 days, nine per cent of the solitary cells had divided, and DNA measurements showed another 20% to have entered the S phase. About 40% of the initial single cells presented morphological alterations after the stimulation which are known to be early signs of entrance into the cell cycle after blockage in G1/G0. The present results in combination with earlier findings suggest that "old" cells approaching the non-dividing state become increasingly insensitive to stimulation by growth-promoting factors.