The mouse spot test: results with a new cross.

The fertility and sensitivity of a new cross using male PDB (a/a b/b d/d p/p) and non-agouti female NMRI (a/a c/c) mice have been assessed. This cross results in embryos which are heterozygous at the same coat colour loci as are present in the embryos from the classical TxC57Bl/6 cross, commonly used in the mammalian spot test. The fertility of this new cross reflects the good breeding performance of the NMRI females. The typical litter size at the age when the F1 are examined for coat colour mosaics is 9-10, and this, together with the high pregnancy rate, results in substantial economies in the number of treated animals. The sensitivity of the cross to treatment with ethylnitrosourea, procarbazine, isoniazid, cyclophosphamide and 4-nitroquinoline oxide has been shown to compare well with other published results. If further evaluation of this cross confirms these preliminary results, then the cross can be used to reduce numbers of experimental animals without loss of sensitivity.