The effect of N-acetylcysteine (NAC) on morphological and physiological properties of "normal" 3T3 and 3T3-SV40 fibroblasts was studied. Incubation of the cells with 10 and 20 mM NAC for 20 h resulted in a reversible increase in the intracellular level of reduced glutathione and disorganization of actin cytoskeleton. Surprisingly, upon removal of NAC, 3T3-SV40 fibroblasts demonstrated formation of well-adhered cells with structured 3T3-like stress-fibers. Neither changes in glutathione levels, nor cytoskeleton disorganization/assembly abolished resistance of 3T3 cells to invasion by the bacterium Escherichia coli A2. On the other hand, pretreatment with NAC converted bacteria-susceptible 3T3-SV40 cells into resistant ones. These results show that NAC can induce partial reversion of transformed phenotype. We suggest that this effect is due to NAC-induced modifications of cell surface proteins rather than to changes in the level of intracellular glutathione.