The composition of human beta-crystallins displayed specific changes with age and region of the lens. 27 kD and 29 kD human beta-crystallin subunits were singled out for study. The 29 kD beta-crystallin subunit constituted approximately 10% of the total lens crystallins at 8 months of fetal life. Its accumulation decreased steadily to 3.3% during postnatal year 1, to 0.5% by year 5 and to 0.3% thereafter. At all postnatal ages, however, it persisted mainly in the superficial fibers. Thus in a 17-years old lens it made up 1.3% of the superficial fiber soluble protein but was already absent from deep cortical and nuclear fibers. The 27 kD subunit increased steadily from 3.5% at 8 months fetal to 7% at year 5; it then decreased steadily to 1.2% in the 86-year old lens. It persisted in all regions of the lens but decreased markedly in the deep cortical and nuclear fibers with increasing age beginning at 5-17 years of age. Studies on the oligomeric structure of human beta-crystallin must take into account age-related changing quantitative patterns in the subunit polypeptide composition of this lens protein.