A neomycin resistance gene cassette was used to disrupt the constant gene segment (designated C-delta). Flow cytometry showed that cells isolated from the lymphoid or epithelial organs of homozygous mice did not react with an anti-pan Tcrd mAb. Immunoprecipitation showed an absence of expression on the surface of thymocytes isolated from homozygous mutant embryos. (J:3865, J:91942)