Recent immunoaffinity studies demonstrate two populations of high density lipoprotein (HDL) particles: one contains both apolipoprotein (apo) A-I and A-II [Lp(A-I w A-II)], and the other contains apoA-I but not A-II [Lp(A-I w/o A-II)]. To investigate whether these two populations are derived from different precursors, we applied sequential immunoaffinity chromatography to study the lipoprotein complexes in HepG2 conditioned serum-free medium. The apparent secretion rates of apoA-I, A-II, E, D, A-IV, and lecithin:cholesterol acyltransferase (LCAT) were 4013 ± 1368, 851 ± 217, 414 ± 64, 171 ± 51, 32 ± 14, and 2.9 ± 0.7 ng/mg cell protein per 24 h, respectively (n = 3-5). Anti-A-II removed all apoA-II but only 39 ± 5% (n = 5) apoA-I from the medium. These HepG2 Lp(A-I w A-II) also contained 31 ± 1% (n = 5) of the apoD and 82 ± 2% (n = 3) of the apoE in the medium. The apoE existed both as E and E-A-II complex. Lipoproteins isolated from the apoA-II-free medium by anti-A-I contained, besides apoA-I, 60 ± 3% of the medium apoD and trace quantities of apoE. The majority of HepG2 apoA-IV (78 ± 4%) (n = 3) and LCAT (85 ± 6%) (n = 3) was not associated with either apoA-I or A-II. HepG2 Lp(A-I w A-II) contained relatively more lipids than Lp(A-I w/o A-II) (45 vs. 37%). The phospholipid, free cholesterol, esterified cholesterol, and triglyceride weight ratios in Lp(A-I w A-II) and Lp(A-I w/o A-II) were 57:27:6:10, and 54:22:7:17, respectively. Electron micrographs showed both discs and spheres in each particle type, but more discs were seen in particles with apoA-II. Upon nondenaturing gradient gel electrophoresis, 66-85% of the Lp(A-I w A-II) was located in the 9.2-17.0 nm Stokes' diameter region. In contrast, 74 ± 14% of the particles without apoA-II were located in the size region below 8.0 nm. These studies showed that 24-h HepG2 conditioned medium contained Lp(A-I w A-II) and Lp(A-I w/o A-II) with physical-chemical characteristics similar to nascent HDL. These nascent apo-specific lipoproteins may be precursors of their plasma counterparts.