Cylindrospermopsin (CYN) is a hepatotoxic alkaloid found in the blue-green alga Cylindrospermopsis raciborskii (C. raciborskii). Data indicating CYN alone does not account for the toxicity of freeze dried cultures of C. raciborskii have been presented recently. In an attempt to explain these data, we have purified and characterized the structure of an analog of CYN, deoxycylindrospermopsin (deoxy-CYN). Three mice dosed intraperitoneally (IP) with 0.8 mg/kg of deoxy-CYN showed no toxicity after 5 days. Comparison with the toxicity of CYN (5 day median lethal dose approximately 0.2 mg/kg IF) and its relative abundance in C. raciborskii suggest deoxy-CYN does not contribute significantly to the toxicity of C. raciborskii. The additional toxicity of freeze dried C. raciborskii over pure CYN, therefore, remains unexplained. (C) 1999 by John Wiley & Sons, Inc.