The present study examined the effects of aging and cataractogenesis on the biochemical properties of the uniquely heat-stable lens crystallin, beta basic principle polypeptide (βBp). Using the techniques of SDS-polyacrylamide gel electrophoresis (SDS-PAGE) and Western-blot immunoassay, we analyzed cortical and nuclear lens sections from normal lenses of individuals aged 0 to 91 years forβBp content in the soluble fraction, and retention of heat stability with aging. In addition, we compared the characteristics ofβBp in cataractous lenses with those of normal lenses of approximately the same age. WhileβBp is synthesized in new cortical cells throughout life, theβBp of the nucleus, which had been laid down early in life, decreased significantly in both absolute concentration and in its proportion of the total soluble protein fraction during the normal aging process. In addition, posttranslational changes in the nuclear solubleβBp result in a gradual loss of approximately 3000 d in the apparent mass of the gBp molecule; i.e., as a result of the aging process, the single heat-stable band of an apparent mass of 26 kd on SDS-PAGE of the young lens nucleus becomes two heat-stable bands, one at 26 kd and one at 23 kd. Normal lenses up to 91 years of age always retain some of the 26 kd subunit, whereas lenses with severe nuclear cataracts had only the 23 kd subunit in the soluble fraction.