The effects of schedule and sequence on the survival of EMT6 tumor cell and bone marrow (CFU-GM) obtained after treatment using combinations of cyclophosphamide (CTX) and thiotEPA or melphalan (L-PAM) were examined and analyzed by isobologram methodology. On a single-injection schedule, when CTX and thio-tEPA were given simultaneously or thiotEPA was given prior to CTX, the result was slightly greater than additive tumor-cell kill. However, when CTX preceded thiotEPA by 4 h, there was less than additive cell kill. When the interval between the administration of the two drugs was 8 h, both sequences of the drugs produced greater than additive tumor-cell kill. Simultaneous administration of CTX and thiotEPA on a multiple-injection schedule resulted in sub-additive tumor-cell kill. On the multiple-injection schedule, extending the interval between injections of CTX and thiotEPA to 4 and 8 h resulted in increasing tumor-cell kill. With the 4- and 8-h intervals, no significant sequence-dependent difference in tumor-cell kill was obtained. The results of CTX and L-PAM combinations paralleled those of CTX and thiotEPA. Bone marrow (CFU-GM) survival was used as a representative normal tissue with which to compare tumor-cell survival after each treatment to obtain a measure of therapeutic effect. The trends for the ratios of bone marrow: tumor cell survival were the same for the treatment sequences of CTX with thiotEPA or L-PAM; however, greater magnitudes of differential tumor-cell kill were obtained with CTX L-PAM combinations. Using this measure, the greatest therapeutic effectiveness was seen with single-dose L-PAM or thio-tEPA followed 4 h later by CTX and with CTX given as a single or as multiple doses followed 8 h later by L-PAM or thiotEPA. Such data from tumor-model systems may be useful in the development of more effective alkylating agent regimens for use in the clinic.