The toxicity of isopropyl myristate (IPM) to microorganisms varies from lot to lot and between manufacturers. The toxicity is not directly proportional to the number and level of IPM homologues present nor to the small quantities of fatty acids, alcohols, aldehydes, or ketones found in both filter- and heat-sterilized IPM. The toxicity of IPM, both filter- and heat-sterilized, has no correlation with the usual physical and chemical characteristics of IPM but shows excellent correlation (correlation coefficient, r = 0.88) with the pH of a water extract of IPM. The toxic compounds can be removed and the toxicity of both filter- and heat-sterilized IPM can be reduced by basic alumina treatment. These findings may indicate that trace amounts of acidic catalysts remaining after production of IPM are responsible for the toxic effect.