The in vivo development of radiation- and doxorubicin-induced resistance was studied in a chemosensitive and radiosensitive human neurogenic sarcoma (Essen neuroectodermal tumor line 2) xenografted in nude mice. Dose-response curves were generated for the parent tumor line, and growth delay (GD) and specific growth delay (SGD) were the study end points. An intravenous injection of doxorubicin at 10 mg/kg, the lethal dose for 10% of the study population (LD10) in nude mice, and a single dose of 12 Gy radiation were determined to be isoeffective and were thus maintained for all subsequent treatments. For the induction of resistance to both treatment modalities, regrowing tumors were transplanted into successive generations of nude mice and retreated. This procedure was repeated 13 and 9 times, respectively, for the doxorubicin and radiation treatments. The response was monitored in all passages. As compared with the parent tumor line, a 50% decrease in SGD was observed following 3.9 and 8.5 treatments with doxorubicin and radiation, respectively. Following four treatments with doxorubicin, SGD in tumors crossed over to radiation therapy declined by 50%. Radiation therapy, on the other hand, caused significant reductions in GD and SGD in tumors that were subsequently exposed to doxorubicin, but it did not induce a 50% decline in response. Overexpression of P-170-glycoprotein was not observed for either treatment modality. The data suggest that treatment with doxorubicin or radiation can potentially induce resistance to subsequent continued or crossover treatment and that this resistance develops gradually. The lack of P-170-glycoprotein over-expression in the resistant cell lines indicates the existence of alternative pathways that may lead to resistance.