The water-soluble polysaccharides from Cassia angustifolia L. leaves were isolated and fractionated. The acidic polysaccharide fraction was separated into two subfractions S1 und S2 consisting of L-rhamnose, L-arabinose, D-galactose, and D-galacturonic acid. Further fractionation of the predominant S1 by GPC gave two fractions S1A and S1B with an average molecular weight of 2 × 106 and 1.5 × 105 d, respectively. Methylation analysis of S1A showed the presence of 1,4-linked galacturonic acid (31.0%), 1,2-linked rhamnose (14.5%), 1,2,4-linked rhamnose (15.8%), 1,3,6-linked galactose (15.3%), smaller amounts of 1,3-linked arabinose, 1,5-linked arabinose, and terminal galactose and arabinose residues. Mild acid hydrolysis of S1A indicated that the backbone consists of 1,4-linked galacturonic acid and 1,2-linked rhamnose residues in the ratio of 1:1. Every second rhamnose is connected via C-4 to arabinogalactan sidechains. The antitumor activity of the polysaccharide fractions was tested against the solid Sarcoma-180 in CD1 mice. Only S1A exhibited a significant antitumor activity with an inhibition rate of 51%.