Surgical removal of the bursa of Fabricius from newly hatched chicks resulted in a depletion of immunoglobulin A (IgA) from serum and bile of 55 and 67% of the birds, respectively, up to 11 weeks of age. The occurrence of IgG and IgM in serum and IgG in bile was not affected by neonatal bursectomy (Bx). A slight compensatory increase in the occurrence of IgM in bile was noted in Bx birds. When exposed to a lentogenic strain of Newcastle disease virus by either intramuscular or intratracheal routes, the Bx group produced a level of serum hemagglutinating antibody and tracheal wash neutralizing antibody indistinguishable from that reached in the sham-Bx group. All of the vaccinated Bx birds challenged by intratracheal exposure to a velogenic strain of Newcastle disease virus remained healthy. These results suggest that IgA is not essential for the development of immunity in the chicken and that other locally produced immunoglobulins or transuded serum antibody may protect the tracheal mucosa in the absence of IgA.