The present paper describes lymphomagenesis in Swiss mice given nitrosomethylurea (NMU) at birth, and thymectomized at 12 days of age. In mice, at this age, migration of T lymphocytes to other parts of the body has already taken place. On the other hand, the precocious thymectomy prevents any possible interference between thymus and the extrathymic lymphoid system in lymphomagenesis since the major target organ for NMU lymphoma induction is absent. No lymphatic tumours developed in NMU-treated thymectomized mice, although they survived for a long time after thymectomy and about twice as long as the time required for thymus lymphoma production. It can be concluded that NMU in the present experimental system induces lymphomas exclusively in the thymus, while the thymus-dependent and independent areas of the peripheral lymphatic tissues are not affected.