The messenger RNA for a beta-crystallin polypeptide with a molecular size of 27 kilodaltons, first detected 5 to 10 days after birth in the normal mouse lens and the Nakano mouse cataract, was not detected in the Philly mouse cataract with translation in vitro. The heterozygous Philly lens had intermediate levels of the 27-kilodalton beta-crystallin polypeptide and exhibited delayed onset of the cataract. The deficiency of functional 27-kilodalton beta-crystallin messenger RNA is the earliest lesion reported yet for the Philly lens and points to a transcriptional or posttranscriptional developmental defect in this hereditary cataract.