We recently described an unknown apolipoprotein that is present on the lipoprotein particles isolated from regenerating rat sciatic nerves. In the regenerating nerve, the concentration of this apolipoprotein rises 500-fold over its concentration in the normal nerve. In this report we have identified the apolipoprotein by partial amino acid sequence analysis as apolipoprotein (apo) D. Characterization of rat apoD by sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed it to be composed of a series of molecular weight isoforms of between 27 kDa and 31 kDa that increase 2 kDa in apparent molecular mass upon reduction. Rat apoD has multiple isoelectric points between pH 4.05 and 4.37, apparently resulting from N-linked glycosylation. In the rat, unlike the human, little apoD is found in plasma. However, immunocytochemical localization of apoD in 12 tissues (liver, kidney, bladder, adrenal, cerebrum, duodenum, testis, lung, spleen, pancreas, heart, and skin) showed that a variety of cells contained substantial levels of apolipoprotein. The broad distribution of apoD suggests that it may play a general role in cellular metabolism. Moreover, many of the same cell types varied dramatically in their content of apoD in different tissues, suggesting that the uptake or secretion of apoD by cells is regulated.