The activity of 5-aminolevulinic acid synthetase (δ-aminolaevulinate synthase, EC 2.3.1.37) in achlorophyllous peanut cells in suspension culture was determined in the presence of 1 mMlevulinic acid. It never exceeded 10% of the activity in green tissue. A feedback inhibition by protoheme may be the cause of low enzyme activity. The concentration of protoheme was 10- to 20-fold greater in these cultured cells than in leaves. The pool of protoheme was shown to act as a reservoir for peroxidase synthesis. This synthesis continued for 48 h in the presence of levulinic acid. The heme prosthetic group of peroxidase was only slowly labelled during the feeding with [14C]5-aminolevulinic acid.