Immunocytochemistry fails to detect expression of the lens major intrinsic protein (MIP) in 16day embryos of the congenitally cataractous mouse, CAT, which inherits a dominant mutation assigned to the distal end of mouse chromosome 10.In situhybridisation, however, detects MIP mRNA in CAT embryo lens fibre cells at a level approximating 60% of that detected in embryonic lens fibres of the non-cataractous MF1 mouse. Northern blot hybridisation reveals that the most abundant MIP mRNA transcript in the adult CAT lens is truncated when compared to that in the adult MF1 lens. The results are consistent with a cataractogenic mutation in the mouse gene for MIP (Mip) which has also been mapped to the distal end of mouse chromosome 10.