SummaryRadiotherapy seems to have arrived at the limit of its therapeutic possibilities in the treatment of solid malignant tumors.At present, the association of radiotherapy and cytotoxics has not given the results which might have been hoped for, from experimental work. We believe that it is due to the fact that the modalities of administration of cytotoxics in conjunction with radiotherapy not have been determined.We have followed the action of alkylating agents, antimetabolites, plant alkaloids and antibiotics combined with X-rays on the variation of the number of nucleated cells of the bone marrow.We were not able to show a potentialization effect but just to show an additive effect of varying degrees.However, we have been able to classify this cytotoxics into drugs with a weak and short-termed action, and those with a more intense and long-termed action.In general, the alkylating agents belong to the first group, except for myleran; the others belong to the second.We were able to conclude that in the case of association with X-rays, the drugs of the first group must be repeated daily, and those of the second group only every three days.One is able to give an important local dose of radiation to a tumor and increase the action of this dose by a cytotoxic administered at a minimal dose.A radioresistant tumor can be cured by this association therapy.We were also able to show that the association of a cytotoxic of the first group with one of the second group has the same effect as an association of X-rays and a cytotoxic of the second group.The association of cytotoxics alone can only be preferred in exceptional cases, for its effect on the economy and immunity is more harmful.In cases of the most radioresistant tumors, association of two cytotoxics with different actions and X-rays can be very useful.