Lipoprotein(a) (Lp(a)) is an atherosclerosis-causing lipoprotein that circulates in human plasma as a complex of low density lipoprotein (LDL) and apolipoprotein(a) (apo(a)). It is not known whether apo(a) attaches to LDL within hepatocytes prior to secretion or in plasma subsequent to secretion. Here we describe the development of a line of mice expressing the human apo(a) transgene under the control of the murine transferrin promoter. The apo(a) was secreted into the plasma, but circulated free of lipoproteins. When human (h)-LDL was injected intravenously, the circulating apo(a) rapidly associated with the lipoproteins, as determined by nondenaturing gel electrophoresis. Human HDL and mouse LDL had no such effect. When h-VLDL was injected, there was a delayed association of apo(a) with the lipoprotein fraction which suggests that apo(a) preferentially associated with a metabolic product of VLDL. The complex of apo(a) with LDL formed both in vivo and in vitro was resistant to boiling in the presence of detergents and denaturants, but was resolved upon disulfide reduction. These studies suggest that apo(a) fails to associate with mouse lipoproteins due to structural differences between human and mouse LDL, and that Lp(a) formation can occur in plasma through the association of apo(a) with circulating LDL.