Experiments were undertaken to confirm and elucidate the significance of the observation, previously reported by other investigators, that alkaline phosphatase (AP) activity, normally not demonstrable in the thymic lymphoid cells of adult C57BL mice, appears during the development of radiation-induced thymic lymphomas. Histochemical studies revealed faint AP in the thymus of 17-day embryos but none in the thymic lymphoid cells of newborn C57BL mice or in those of older mice up to 180 days of age. Thymic AP did not appear in response to adrenalectomy or the administration of corticosteroids in thymolytic doses, though the latter treatment sharply increased splenic AP activity. Splenic AP activity was also increased in neonatally thymectomized animals that developed the wasting syndrome and in others with spontaneous runting. Histochemical and histopathological studies were performed on consecutive serial sections of samples from the thymuses of C57BL mice killed at various intervals after (a) exposure to whole-body irradiation; (b) thigh-shielded irradiation; (c) intrathymic injection of a passage line of a leukemogenic virus (RLV) originally extracted from a radiation-induced lymphoma; and (d) saline injection. Preliminary studies on the variation of AP activity during serial transplantation of virus-induced lymphomas are also reported. The data support the view that the development of AP activity in murine thymic lymphomas is a specific response induced concomitantly with and directly related to the neoplastic transformation per se, and not merely a reflection of cellular immaturity or a nonspecific response to trophic or involutional stimuli.