Monospecific antiserum obtained from rabbits hyperimmunized against homogeneous p27 group specific protein purified from avian myeloblastosis virus was commercially procured and was then conjugated with fluorescein isothiocyanate. The conjugate was applied to spleens from naturally or experimentally infected chickens that had no evidence of lymphoid tumors. Fluorescence was usually localized in connective tissue of sheathed capillaries giving it a ring-like appearance. Sites of fluorescence corresponded to sites of greatest virus concentration as detected by electron microscopy, indicating that in such cases the group specific antigen may have been associated with virus particles. The group specific antigen could also be detected in the spleen by complement fixation and results of this test usually agreed with the immunofluorescent test and with the phenotypic mixing test which detects exogenous lymphoid leukosis virus.