The origin of murine ovarian teratomas and their use to estimate centromere-to-gene distances is discussed. As an example of how such a system can be used, the distance from the centromere of chromosome 7 to the Gpi-1 locus was determined by analyzing the Gpi-1 phenotypes in 47 ovarian teratomas from Gpi-1a/Gpi-1bfemales. The percent recombination was estimated to be 10.6. It is suggested that ovarian teratomas and parthenotes may be used for half-tetrad analysis.