Pig plasma lipoproteins were separted into four density classes (very low density, two low density and high density lipoproteins, VLDL, LDL1, LDL2 and HDL respectively) from 670 ml plasma by ultracentrifugation in a continuous density gradient using the Spinco Ti15 zonal rotor. LDL1 and LDL2 were partly characterised. LDL1 and LDL2 are beta-migrating lipoproteins of different size and hydrated density; they are similar to human LDL2 and LDL3 respectively. Pig plasma contains about twice as much LDL1 as LDL2. LDL1 migrates at Sf 4.9 (modal value), and has a mean diameter of 217 A and a modal density of 1.035 g/ml (range 1.03-1.04 g/ml). LDL2 migrates at Sf 1.8 and has a mean diameter of 195 A and a density of 1.050 g/ml. Both lipoproteins are precipitated by heparin and Mn++ or by dextran sulphate and Ca++. The apoproteins of LDL1 and LDL2 are both largely insoluble in 8 M urea solution. When dissolved in 1% sodium dodecyl sulphate solution and electrophoresed on polyacrylamide gel at pH 7.0, the apoproteins of LDL1 and LDL2 formed a pattern of multiple bands of high molecular weight similar to that obtained from the apoprotein of human LDL. Both LDL1 and LDL2 share a major antigen with each other and with VLDL; in this respect again they resemble human LDL. The amino acid compositions of LDL1 and LDL2 are very similar. We concluded that the apoprotein moieties of pig plasma LDL1 and LDL2 are probably identical, and similar to apoprotein B in human serum. Zonal ultracentifugation has proved to be a rapid and effective method for isolating large quantities of these two lipoprotein classes for further metabolic studies. This method allows rapid bulk preparation of lipoproteins, and provides a record of their distribution and quantity in a continuous density gradient.