This report contains additional observations on the cell lineage relationships of the motor neurons of the facial nucleus and of the cerebellar Purkinje cells of the mouse. These cell populations were quantitatively analyzed in the mosaic brains of experimental aggregation chimeras. The cell markers used to perform the analysis were (1) staining differences in beta-glucuronidase enzyme activity due to differences at the Gus locus on chromosome 5 and (2) self-destruction of all Purkinje cells of one genotype due to the cell autonomous action of the lurcher gene on chromosome 6. Of 6 animals examined in the Purkinje cell study, 5 showed left/right differences in the clonal composition of the cerebellum. Of 4 animals examined in the facial nucleus study, all 4 showed differences in genotype ratio. These results imply that the event that defines the selection of the progenitor cells for each of these two cell types is, in fact, two bilaterally independent events. This is consistent with the pictures of the early nervous system derived from embryological observations. The implication of these findings for cell mixing during development is discussed.