AbstractThe problem of using OH scavengers, with special emphasis on biological systems, is discussed. The problems arising from inhomogeneity and from possible large gradients in scavenger concentrations between the cell and media, or between different parts of the cell, are analysed, and their implications on scavenging efficiency are examined. The possible reactions of secondary radicals and their effect on scavenging and even on possible sensitization are described. The limitations of the usage of OH scavengers in biological systems are discussed.