SummaryTotal lipids extracted from J‐positive cattle serum, erythrocytes or spleen exhibit J blood‐group activity. The J subsance is concentrated in a lipid fraction obtained by column chromatography. Following mild alkaline hydrolysis or reduction with complex hydrides (LiAlH4, LiBH4), the J activity remains detectable in this lipid fraction even though all acyl ester groups have been destroyed as revealed by ester group determination. This disagrees with the suggestion that fatty acyl esters are essential for J activity. This was confirmed by experiments with a water‐soluble J‐active product prepared by ozone treatment of glycosphingolipids from bovine spleen. The results of these experiments are in favour of a glycosphingolipid containing an unusually lang oligosaccharide chain. Furthermore, it appears that the terminal moiety of the J determinant is not necessarily an N‐acetyl galactos‐amine unit as suggested