Abstract Pentosanpolysulfate (PPS) represents the product obtained after sulfation of xylan and is composed of β 1 → 4- D- xylopyranose residues sulfated at C2 and C3. Studies have shown that this compound can often be effective in relieving the symptoms of interstitial cystitis (IC). In order to elucidate the mode of action of PPS in IC, a sensitive and reliable assay was needed. To this end we prepared an immunogenic form of PPS by coupling it to methylated bovine serum albumin (MBSA). This complex was used to immunize NZW rabbits (1 mg, IM). Four of five animals responded with anti-PPS antibodies, three of which had high titer (>1/2000) as measured by an enzyme-linked immunosorbent assay (ELISA). All sera were routinely absorbed with an MBSA-Sepharose immunoadsorbent to remove anti-MBSA antibodies. ELISA inhibition tests were used to determine the sensitivity and specificity of the sera. At least 50 ng/ml of PPS could be routinely detected by this assay. A number of naturally occurring proteoglycans, polysaccharides, monosaccharides and disaccharides were examined for reactivity with the antibodies but only heparin was an effective inhibitor. Absorption with heparin immunoadsorbents reduced, but did not eliminate, the ability of heparin to inhibit anti-PPS binding. This activity could be destroyed by treatment with heparinase without affecting PPS inhibition. Normal urine did not affect the ELISA or ELISA inhibition tests and thus allowed the determination of PPS levels in IC patient urines. Initial analysis of seven IC patients receiving oral PPS revealed urine concentration of 0.8–16.0 μg/ml. No inhibition could be detected in pre-treatment urine samples.