Mipafox synthesized by one of us (F.C.G.H.) by the method of Heath22, with additional washings and a final vacuum sublimation, has been determined by NMR to be essentially 100% pure. The Mipafox used in the research reported in the body of this paper, purchased from another source12, appears to be less than 50% pure. With the pure material, we now find no evidence of Mipafox hydrolysis by Penicillium funiculosum at pH 4.3 or 7, but continue to find a DFPase-like activity, i. e., DFP hydrolysis, as shown in Figs. 3 and 4. While the DFPase-like activity continues to parallel that of a phosphodiesterase, the parts of this paper purporting to present new findings involving Mipafox should now be regarded with reservation. The inhibition of Mazur type but not of squid type DFPase12, also given in Fig. 2, is still found with the high purity material. Details substantiating this addendum, including quantitative corrections of enzyme kinetics, will be submitted for publication in the near future. We hope that the publication of the addendum will help to correct the questionable parts of the oral presentation of this material.