The synthesis of 23 kDa protein was enhanced when mouse peritoneal macrophages were exposed to oxidative agents such as hydrogen peroxide and menadione, or to sulfhydryl-reactive agents such as diethylmaleate, cadmium chloride and sodium arsenite. After 11 h exposure to these agents the 23 kDa protein was one of the actively synthesized proteins in the macrophages. Under similar conditions the 34 kDa protein previously identified as heme oxygenase, was induced and its synthesis preceded that of the 23 kDa protein. Neither the 23 kDa or the 34 kDa protein was induced by hyperthermia. Conversely, the various oxidative and sulfhydryl-reactive agents employed here did not induce the major heat shock proteins in the macrophages. When the macrophages were activated by bacterial lipopolysaccharide or other stimulants, many proteins are known to be induced, however, the 23 kDa and 34 kDa proteins were not induced. The 34 kDa protein, i.e., heme oxygenase, has been found to be stress-induced in various types of cell, but not the 23 kDa protein. This suggests that the 23 kDa protein is a stress protein predominantly expressed in macrophages.