Plasma apolipoprotein(a) [apo(a)] shows considerable size heterogeneity, existing as discrete glycoprotein isoform variants that range in apparent molecular mass from approximately 400 to 800 kDa. To study the molecular basis of protein size variability, we have isolated liver RNA from individuals with different apo(a) isoforms, and identified apo(a)-specific transcripts using Northern blot analysis. Transcript sizes were shown to be variable (8.0-12 kb) and in all cases were closely correlated with protein masses (590-850 kDa) as determined from immunoblots. Thus, it is almost certain that apo(a) isoform size variation is due to allelic differences in the number of its tandemly repeated sequences of 114 amino acids that resemble kringle four of plasminogen. The high carbohydrate content of apo(a) makes true molecular weight estimations in SDS-PAGE gels difficult. However, a recombinant form of apo(a) containing 17 kringle repeats (calculated molecular mass of 250 kDa) migrates on SDS-PAGE gels only slightly below apoB-100, with an apparent molecular mass of approximately 500 kDa. Since smaller protein isoforms have been observed in the population, this suggests that plasma apo(a) isoforms contain from less than 17 to greater than 30 tandemly repeated kringle units.