Alkylating agent-sensitive and -resistant L1210 leukemia cell lines were used to determine the tumor response to dose levels of drugs that exceeded conventional doses up to a factor of 10. Since those dose levels were lethal to the host mice, tumor response was based on the results of in vivo bioassays of spleen and/or tumor from drug-treated and control mice. When mice bearing about 10(8) drug-sensitive leukemic cells were treated with a single, conventional (approximately 10% lethal) dose of cis-diamminedichloroplatinum, L-phenylalanine mustard (melphalan), or 1,3-bis(2-chloroethyl)-1-nitrosourea, 10(1) to 10(4) tumor cells were recovered by bioassay. Treatment at doses that were 2 to 8 times the 10% lethal dose of either of those drugs resulted in no recoverable cells and survival of all bioassay recipient mice. Mice bearing advanced L1210 leukemia resistant to cis-diamminedichloroplatinum (L1210/DDPt), 1,3-bis-(2-chloroethyl)-1-nitrosourea (L1210/BCNU), cyclophosphamide (L1210/CPA), or melphalan(L1210/L-PAM) also were treated with a 10% lethal dose and greater doses of the drug to which the tumor line was resistant. Bioassay results indicated a direct correlation between dose intensity and tumor cell kill, the response being linear. Similarly, when mice with L1210/BCNU were treated with high doses of N-(2-chloroethyl)-N''-(2,6-dioxo-3-piperidinyl)-N-nitrosourea or 1,1',1''-phosphinothioylidynetrisaziridine (thioTEPA) and when mice with L1210/DDPt were treated with cyclophosphamide, an increasing, linear cell kill resulted throughout the high-dose range. Overall, these results indicate that resistance to these alkylating agents can be overcome by dose intensification and that the tumor response is linear in relation to increasing dose level.