A high frequency of chromosomal non-disjunction occurs spontaneously in mice heterozygous for some Robertsonian translocations. If animals heterozygous for the translocation and homozygous for different alleles of a marker gene are mated together a few young homozygous for the marker arise through non-disjunction, and their frequency can be used as a measure. This method has been used with the Robertsonian translocation Rb(9.19)163H and the marker ruby ru (chr. 19); Rb(4.6)-2Bnr with brown (b) and misty (m) (chr. 4); and Rb(9.14)6Bnr with hairless (hr) and piebald (s) (chr. 14) respectively. The frequencies of marked young were: Rb163 0/5260 ruru; Rb2 21/1997 mm bb; and Rb6 19/1702 hrhr ss, and the corresponding calculated non-disjunction frequencies in each arm of the translocation were Rb163, <5 %; Rb2, 15%; Rb6, 15%. These figures show reasonably good agreement with values obtained by other methods. A search for genetic or environmental factors affecting the frequency of marked young in Rb2 and Rb6 revealed that in Rb2 the frequency increased with maternal age, whereas in Rb6 the maternal age of the marked young was non-significantly below that of the total progeny. The reasons for this discrepancy are not clear.