The enzymatic formation of p-hydroxybenzoic acid from p-coumaric acid has been detected in cell free extracts of Lithospermum erythrorhizon cell cultures. In the absence of NAD, p-hydroxybenzaldehyde is formed instead of p-hydroxybenzoic acid. The reaction is specific to p-coumaric acid. In addition, a p-hydroxybenzaldehyde dehydrogenase activity is also found. The results suggest that the reaction mechanism is non-oxidative.