Several phenolic compounds were examined for their ability to inhibit in vitro antibody production and human and mouse cell clone formation. Those compounds (gallic acid, nordihydroguaiaretic acid, methylparaben, and propyl gallate) which inhibited antibody production also suppressed clone formation. These effects occurred well below toxic concentrations. Other closely related compounds (p-hydroxybenzoic acid, shikimic acid, and vanillic acid) were without either effect. The data suggest that certain phenolic compounds, present either as normal constituents of food or as additives, may alter various cellular functions at or below concentrations found in the gut. © 1981, SAGE Publications. All rights reserved.