Lp lipoproteins have been prepared by a mild method from the serum of a large number of individuals. Approximately 25% of the individuals tested showed the presence of a double Lp peak in analytical ultracentrifuge diagrams. These double peaks were designated Lp(a)-1 and Lp(a)-2 to distinguish them from the single Lp(a) peak. The mean viscosity-corrected sedimentation coefficient, S 1.004, 20 C and density of the single Lp(a) peak were 15.8±1.8s (n=32) and 1.076±0.01 g/ml, of the Lp(a)-1 peak were 13.5±1.1s (n=14) and 1.064±0.007 g/ml, and of the Lp(a)-2 peak were 16.8±1.7s (n=14) and 1.074±0.009 g/ml. Absorption tests using a double and single Lp preparation showed that both Lp peaks in the double variants possess Lp(a) specificity. Evidence is lacking as yet for individual specificities for either Lp(a)-1 or Lp(a)-2. Interand intra-individual heterogeneity among Lp lipoproteins is discussed.