Repeated parent-hybrid parabiosis was performed in mice across the H-2 barrier (DBA/2 and C57BL/6 × DBA/2)F1). In further groups of mice, parabiosis was combined with the i.v. administration of disrupted liver and spleen cell suspensions, azathioprine, and preliminary thymectomy in various combinations. Only minor modification of the rejection process, in the form of some attenuation of the anticipated second-set response following reparabiosis, was obtained. There was no true tolerance in the form of stable union. During repeated parabiosis, the anaemia-polycythaemia phenomenon was either entirely absent or appreciably reduced in magnitude, depending on the degree of sensitization produced by the initial parabiosis. A bilateral anaemia developed in several pairs late in the course of repeated parabiotic union, and prolonged anaemia developed frequently in both parental and hybrid strains following death and surgical separation of their partners. These findings in the parent strain mice suggest a hybrid-anti-parent reaction. Such a reaction may be explained by exceptions to the conventional rules governing parent-hybrid reactions, but indirect evidence suggests the involvement of the phenomenon of allogeneic inhibition.