The semi-quantitative development of lipoproteins in the serum (beta-, pre-beta- and alpha-lipoproteins) from fetal (day 18 of intra-uterine development) to adult age was determined by agarose gel electrophoresis and determination of total lipids. 2. A high proportion of beta- and a very low proportion of pre-beta-lipoprotein is typical of the fetal stage. In adult age, however, the beta-lipoproteins are the smallest fraction, and the pre-beta-lipoproteins, the second-strongest fraction. 3. The transition from fetal to adult lipoprotein pattern is more or less abrupt, with the change of the proportion for each of the three lipoprotein fractions occurring at different intervals from birth. Adaptation of the fetal lipoprotein status to that of the adult animal in regard to quantity and mutual relationship of the fractions was observable, at the earliest, 7 weeks after birth.
