The weaver mutation (wv) causes a near total loss of midline granule cells in the mouse cerebellum. The cellular site of mutant locus action leading to the granuloprival phenotype was examined with experimental intraspecific and interspecific homozygous weaver chimeras. It was found that the granule cells which survived and successfully migrated to the internal granular layer of the chimeric cerebellum were all of the wild-type (non-wv) genotype. Using interspecies chimeras, it was determined that the genotype of Purkinje cells and Bergmann glia cells was apparently irrelevant to the survival of granule cells. It is concluded that granule cell death is most likely due to the wv locus acting intrinsically to the weaver granule cells, and not to another cellular site of gene action.