Previous studies showed that T-lymphoid cells are permeable to the tetracyclines, whereas B-lymphoid and erythroid cells are not. The tetracyclines impair mitochondrial protein synthesis if they have access to cells. Inhibition of mitochondrial protein synthesis during a number of cell cycles results, as a consequence, in proliferation arrest. The tetracyclines can therefore be considered as cytostatics. In the present study the effect of prolonged treatment with doxycycline on the growth of a T-cell type leukaemia of the rat was investigated. It is shown that doxycycline treatment inhibits not only tumour cell proliferation, but leads moreover to complete tumour eradication. The way by which the latter is achieved depends on the doxycycline concentration and, surprisingly, on the stage of tumour progression at which doxycycline administration is started. As, because of the permeability barrier, the proliferation of erythroid and B-lymphoid cells is not affected by the tetracyclines, the tetracyclines may provide a tool without serious side-effects in the therapy of T-type tumours.