A highly sensitive high-performance liquid chromatography method, which was established by us for the determination of chondroitin sulphates in biological substances as their unsaturated disaccharides, was applied to elucidate the qualitative and quantitative differences in chondroitin sulphates in rabbit plasma and serum samples. In this work, it was found that rabbit plasma contains low-sulphated chondroitin 4-sulphate (approximately 40% sulphation at the 4-position of N-acetyl galactosamine), while serum contains the low-sulphated chondroitin 4-sulphate and fully sulphated chondroitin 4-sulphate (approximately 96% sulphation). The latter was released from platelets during coagulation of blood.