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Transgenic mice were produced carrying the coding region of the rabbit κ-casein gene linked to the upstream region of the rabbit whey acidic protein gene. Mice from the highest-expressing line produced 2·5 mg rabbit κ-casein/ml in their milk. The foreign protein was associated with the casein micelles and altered micelle size, though in the high-expressing line rabbit κ-casein also segregated into the whey fraction obtained after centrifuging the milk samples. Milk from transgenic mice had the same overall protein content as that from non-transgenic mice, except for the transgene product. However, litters fed with this transgenic mouse milk grew less well than litters given milk from non-transgenic mice. This reduction in growth was not related to changes in mammary gland structure or mammary cell morphology. Preliminary results indicated that milk from the transgenic mice had a higher viscosity.