Transfer of maternal immunoglobulin G (IgG) to the yolk and nestling was investigated in the budgerigar. Specific antibodies to avian polyomavirus and Newcastle disease virus could be detected in 82% of yolk extracts of eggs from seropositive hens. Using a double immunodiffusion assay with anti-chicken IgG antibodies, IgG could also be detected in yolk supernatants with virus neutralizing activity. In all assays, IgG concentrations in the yolk extracts were significantly less than those of the adult budgerigar serum. No antiviral activity was detected in nestling serum. Examination of nestling serum with the double immunodiffusion assay and an immuno-dot-blot technique specific for IgG showed that detectable concentrations of IgG are not present in nestling serum until after the yolk sac is fully absorbed. This observation, coupled with the absence of specific anti-viral antibody in nestlings of seropositive hens, indicated that none of the yolk sac antibody reached the nestling circulation.