Eight human fetal lenses, selected on basis of normality, of a gestational age of 119 to 231 days were analyzed by thin-layer isoelectric focusing (IEF) in agarose/polyacrylamide gels. This method was adapted for the separation of lens crystallins into HM-, alpha-, beta- and gamma-crystallins. It is especially suitable for analysis under non-denaturing conditions of high-molecular-weight crystallins and of insoluble crystallins (WI) solubilized in formamide. The latter could be separated into HM-, alpha- and gamma-crystallins. During fetal development, a considerable increase of gamma-crystallin proportion was observed due to new synthesis. This increase was balanced by a decrease of alpha-crystallin proportion.