The title compound is one of a large number of compounds related to the drug suramin which have been found to have anti-HIV properties. However, its toxicity to uninfected cells is high. We crystallized the compound, determined its structure, and investigated the molecular basis for its high toxicity. The crystals are monoclinic,C2/c,a= 28.614(1),b= 10.031(0),c= 24.051(1) Å, &bgr; = 98.36(2)°,Z= 8, convergence atR= 0.066 for 4264 reflections withI> 2&sgr;(I). The oxygen atoms of one of the sulfonate groups are disordered, as are several water molecules. Stereochemical comparisons with a segment of B-DNA have yielded a plausible explanation for the molecule's toxicity.Key words: crystal structure, cellular toxicity, stereochemistry, NF-03