Erbstatin (ERB), a specific inhibitor of tyrosine protein kinases (TPK) was used to examine the effect of TPK inhibition in X-irradiated cells of two sublines of murine lymphoma L5178Y. These sublines, LY-R and LY-S, differ in their sensitivity to ionizing radiation, UV-C radiation and oxidants. Total tyrosine protein kinase (TPK) activity was higher in unirradiated LY-R than in LY-S cells, whereas its stimulation by irradiation was maintained longer in LY-S than in LY-R cells. Relative cell number measured 24 h after ERB treatment and post-treatment cell cycle distribution did not differ between the sublines. Longer growth observations and flow cytometry revealed higher susceptibility of LY-R cells to ERB as compared to LY-S cells. The inhibitor induced a marked apoptosis in LY-R cells, but only slight apoptosis in LY-S cells, whereas ERB-treated and X-irradiated cells exhibited increased sub G1 fraction and DNA degradation pattern typical for apoptosis, when examined 24 h after treatment. The relative growth rate of LY-R cells subjected to combined (5 mg/ml ERB+X-radiation) treatment was higher than that of irradiated cells during the first week of post-irradiation incubation. This protective effect was negligible in the radiosensitive LY-S cells. On the contrary, with cloning efficiency as the end-point, the effect of combined treatment was additive. We concluded that ERB caused an increase in cell death by apoptosis, and this was reflected in the post-irradiation growth kinetics. However, the altered death mode did not affect the overall lethal effect of X-rays.