Different classes of rabbit serum lipoprotein were prepared by ultracentrifugal flotation at densities 1-006, 1-063 and 1-21 g/ml. Agarose gel electrophoresis on rabbit whole serum and the serum fractions with different densities showed that this technique separates the different lipoprotein classes reasonably well. The electrophoretic mobility of the different lipoprotein classes of rabbit serum seems to be similar to that of the human lipoproteins, with the exception of alpha1-lipoprotein which had a greater mobility than human alpha1-lipoprotein. The chemical composition of rabbit high density lipoprotein (HDL)p was fairly similar to that of human HDL although the former seems to be richer in triglycerides. HDL was, after isolation by ultracentrifugal flotation at density 1-21, delipidated and submitted to gel filtration on Sephadex G-200 in 8 M urea. The major protein fraction of rabbit apo HDL corresponds in elution volume to that of the major fraction of human apo HDL, apoA-I. A protein fraction corresponding to human apoA-II does not seem to be present in rabbit HDL in demonstrable amounts. The rabbit protein fraction sometimes appearing in the area corresponding to human apoA-II could not be found to be affected by the reduction and alkylation method after which human poA-II splits into two identical chains.