In the protein-deficient media plasminogen stimulated the vital functions of cells and in concentrations 10(-7)-10(-10) M it protected cells of sympathetic ganglia, neocortex and continues cell lines under damaging actions of H2O2 (0.0001 M), NH4CI (0.01 M) and cooling. Streptokinase essentially influenced the mode of damaging effect of ATP(0.001 M). Even a short-term exposition (20 min) of PC12 cells with both proteins (each in the concentration 10(-9) M) led to sharp alterations in intracellular ATP- or Ca(2+)-activated proteolysis. In some cases plasminogen and streptokinase provided acceleration of cultured tissue maturation, improvement of cell adhesion, high survival rate, the increase in quantity and length of processes and their arborisation. Electronic microscopy established the character of structural rearrangements of nervous tissue cells (neurons, astrocytes, oligodendrocytes), reflecting the protective action of plasminogen and streptokinase. In the presence of plasminogen and especially streptokinase, the total number of cultured glioma C6 and neuroblastoma IMR-32 cells, the intracellular contents of protein, RNA and DNA increased several-fold. Addition of plasminogen promoted formation of processes by neuroblastoma cells, this suggests initiation of differentiation of cellular elements. In cultures of sensitive and sympathetic ganglia streptokinase increased proliferation of Schwann cells. These proteins did not cause transformation of PC12 enterochromaffine cells to neurons, though plasminogen facilitated it. Plasminogen addition to cell cultures did not increase fibrinolytic activity of the culture medium in the culture medium, and streptokinase did not lose its plasminogen-activating capacity.