Various degrees of therapeutic efficacy, as measured by median survival time and proportion of tumor-free animals surviving to 90 days, were observed against mouse plasma cell tumor LPC-1 with certain combinations of 4 effective drugs administered dually or singly. The most effective treatment was 1,3-diallylurea plus 5-fluorouracil given sequentially; next was 5-fluorouracil; considerably less effective were the others, which included cyclophosphamide and aniline mustard. Subsequent to chemotherapy, the groups of surviving mice were challenged with successive doses of 103 , 104 , and 105 LPC-1 cells. The greater the efficacy of the original treatment, the greater the resistance of the group to subsequent challenge. These results suggest that, after chemotherapy, the degree of resistance to challenge by tumor cells is related both to the direct action of the drugs on the tumor cells and to drug influence on the host defense mechanisms.