Lipase preparations were treated in vitro with 100 chemicals of various classes, many of which are environmental pollutants, to determine their effect upon enzyme activity. The greatest inhibition was caused by mercuric ion and certain heavy metal cations; almost as inhibiting were pentachlorophenol, dicofol, 2,4,5-trichlorophenol, and DDT. Less inhibition was found with other inorganic salts, organo-metals, other organic chemicals, and some pesticides. Apart from dichromate ion, the anions studied had a low to negligible effect on lipase. A mixture of 10 inorganic salts had an approximately cumulative inhibitory effect on enzyme activity. None of the chemicals studied caused a measurable activation of the enzyme.