We investigated the susceptibility of peritoneal mouse macrophages and macrophage and microglia cell lines to the peroxidative activity of beta-haematin, the synthetic polymer identical to native malaria pigment. The extent of lipid peroxidation, measured as production of thiobarbituric acid reactive substances (TBARS), was greater for peritoneal macrophages than for cell lines and microglia cells. TBARS production apparently was not attributable to the release of free iron from the protoporphyrin moiety, but related to lower glutathione content and different lipid composition of the cell membrane. These findings offer a new interpretation for the contentious immunomodulatory effects of beta-haematin reported for phagocytes of different origins.