An outbred stock of Swiss:SE mice has been surveyed for genic heterogeneity at 12 loci, encoding biochemical polymorphisms in mice. Using horizontal starch-gel electrophoresis, 6 loci (Es-1, Es-2, Es-3, Es-5, Trf, Dip-1) revealed no genic heterogeneity within the total sample of 289 male mice examined. The other 6 loci (Mpi-1, Mup-1, Hbb, Gpi-1, Pgm-1, Ldr-1) each showed a 2-allelic variation within each of the 3 stock units (pavillions) involved. A pavillion effect on the observed genotype numbers was found for Pgm-1 and Gpi-1 (P less than 0.10). Inadequate genotyping may have occurred at Gpi-1 and Ldr-1, as suggested by a 'week effect' on the observed genotype numbers (P less than 0.05). For studying long-term genetic changes within outbred stocks, a routine monitoring procedure using biochemical polymorphisms is recommended.