The pollen of rye (Secale cereale) was shown to contain a biologically highly active family of glycosides called the secalosides. Secalosides A and B (1), both of molecular formula C₄₆H₅₁NO₂₄, were found to be epimeric esters of (2-oxo-3-indolyl)acetic acid (4). They are made up, in addition to this heterocyclic aglycon I (4), of three hexose building blocks and a carbocyclic aglycon II, which is an indan-derived dicarboxylic acid (5). In aqueous solution, secalosides A and B interchanged by epimerization at the chiral center of 4. A further epimeric pair, secalosides C and D (2), contain one additional glucose building block. Secalosides A and B, the racemic aglycon I (4), and 2-oxo-1,2,3,4-tetrahydroquinoline-4-carboxylic acid (3), which results from 4 by hydrolytic rearrangement, exhibited significant antitumor activity against S180 sarcoma in vivo. IC50 values obtained were about 5 μg/mouse for the secalosides and 1 μg/mouse for 3 and 4.