Single doses of 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea caused transient suppression of [3H]thymidine incorporation into DNA in bone marrow and gastrointestinal mucosa and more prolonged inhibition of such incorporation in B16 melanoma. A single dose of 1-(2-chloroethyl) -3- (trans-4-methylcyclohexyl) -1-nitro-sourea, 16 mg/kg, doubled the mean life-span after treatment of C57BL times DBA/2F1 male mice bearing 12-day-old B16 melanomas. Subsequent doses timed to minimize toxicity and maximize antitumor effect, however, produced no further prolongation of survival, and studies with B16 melanoma previously expsed to 1-(2-chloroethyl)-3-(trans-4-methylcyclohexyl)-1-nitrosourea demonstrated that the suppression of [3H]thymidine incorporation into DNA was no longer prolonged beyond that seen with normal host tissues. The loss of clinical efficacy was accompanied by a loss of differential suppression of [3H]thymidine incorporation into DNA between the tumor and host tissues.