Estrogen treatment of medaka leads to accumulation of ascites, in which vitellogenin (Vg) and choriogenins (precursors to vitelline envelope) are abundant. Besides those female-specific proteins, we detected a new component in ascites that cross-reacts with antiserum against egg yolk proteins. We tentatively named it egg yolk-related protein (YRP). YRP was purified from ascites by hydroxylapatite chromatography followed by gel filtration. Purified YRP had a molecular mass of 460 kDa in intact state while 570 kDa for Vg. The molecular weight of purified YRP on SDS-PAGE under both reducing and nonreducing conditions was 130 kDa. YRP was confirmed to be a lipoglycophosphoprotein by staining with Sudan black, periodic acid-Schiff (PAS) and methyl green. Amino acid composition of YRP resembled that of Vg except for a relatively low content of serine. A specific antiserum against YRP was raised in a rabbit. Antiserum against YRP specifically immunostained its antigen but not Vg or choriogenins. YRP was detected as a female-specific protein in serum of breeding medaka. The antiserum also cross-reacted with a band at 29 kDa in egg extracts, which is not immunoreactive to antiserum against Vg. These data show that YRP is a precursor to some egg yolk proteins with differing antigenicity from Vg (Hamazaki et al. '87). We thus conclude that YRP is a second form of medaka Vg and rename YRP as Vg 2 while formerly reported Vg as Vg 1.