Transglutaminases, calcium-dependent thiol enzymes, may be involved in cellular growth control and differentiation, having an intracellular regulatory role in some post-translational modifications found in various classes of proteins. In order to elucidate the involvement of this class of enzymes in cellular differentiation processes, we have assayed transglutaminase activity in primary and subcultured rat glial cells. Reduced activity was found from 3rd to 5th passage. In the 5th passage the activity was some 50% of that found in the primary cultures and was not restored by addition of 10 microM retinoic acid. The decrease of TGase activity, observed during serial passages, could represent an early metabolic alteration related to cell dedifferentiation and loss of growth control. In fact, the subcultured cells may have undergone a "disarranged" state, as confirmed by a decrease in GFAP-stained cells and glutamine synthetase activity, respectively, immunocytochemical and biochemical markers of astroglial cells.