The major apolipoproteins associated with oil-storage bodies have been isolated from the mature seeds of six different species of the family Cruciferae. The apolipoproteins were all of molecular mass 19-20 kDa. They were highly abundant in mature seed tissue, accounting for up to 20% total seed proteins, and were localized exclusively on the membranes of oil-storage bodies. Antibodies were raised in rabbits and mice against the six purified apolipoproteins. In each case, the antibodies specifically recognized 19-20 kDa polypeptides on immunoblots of total seed proteins from 15 different species of the Cruciferae. The extent of the immunological cross-reactivity among the six purified seed apolipoproteins of the Cruciferae was investigated quantitatively using enzyme-linked immunosorbent assay. Very high levels of cross-reactivity were obtained, in contrast to a complete lack of cross-reactivity observed when the major seed apolipoprotein of a non-crucifer, Glycine max, was assayed. Peptide mapping studies showed that the different crucifer seed apolipoproteins gave rise to similar proteolytic cleavage products following treatment with Staphylococcus aureus V8 protease, Lysobacter enzymogenes Lys-C endoprotease, and trypsin. The patterns of immunogenic proteolytic cleavage products of the different apolipoproteins were also similar. We propose that there is a family of abundant 19-20 kDa apolipoproteins in mature seeds of oil-bearing Cruciferae. These apolipoproteins are all major components of the membranes of oil-storage bodies. The apolipoproteins are therefore very closely related with respect to their structure, function, and immunological properties.