Chimeric mice were produced by aggregating 2 embryos, each of which was homozygous for a different structural allele of the enzyme beta-glucuronidase. The alleles used were Gusb, the 'wild-type' allele, and Gush, an allele whose gene product shows decreased activity in all tissues as well as decreased heat stability. Staining of untreated adult chimeric livers for glucuronidase activity revealed a mosaic of high (Gusb) and low (Gush) activity cells. The boundaries between cells of different activity were sharp and revealed no diffusion of enzyme or reaction product. Treating sections at 73 degrees c before staining led to a decay of staining activity in normal (non-chimeric) Gush/Gush and Gusb/Gusb tissue. The rates of activity loss under the conditions used differed by 10-fold. Of the 2 genotypes in the chimera, the dark-staining, Gusb, cells decayed in a fashion similar to that of the Gusb/Gusb control. The light-staining, Gush, cells of the chimera lost their staining a a unique fashion. Within 20 min they quickly lost a majority of their staining activity but that which remained was relatively heat stable. The second Gusb-like phase of the decay, seen both photographically and photometrically, suggests that Gusb gene product has been transferred to cells of Gush/Gush genotype.