Soluble immune response suppressor (SIRS), a product of murine Ly-2+ T lymphocytes, is activated to SIRSox by H2O2 produced by macrophages: SIRSox directly inhibits cell division by normal and neoplastic cells and antibody secretion by B lymphocytes. To examine the mechanism of SIRSox-mediated inhibition, a variety of cellular functions were measured after treatment of cells with SIRSox. These included respiration, glucose transport, microtubule content, glutathione content, production of H2O2 or superoxide anion, and the activities of a variety of different enzymes. Several cellular activities or measurements were inhibited or lowered after SIRSox-treatment, including cell division, microtubule content, glutathione reductase activity, and thioredoxin reductase activity; inhibition was partially reversed by the sulfhydryl reducing agent dithiothreitol. Protein sulfhydryl content of P815 mastocytoma cells and several other cell types was lowered by 35 to 45% after exposure to SIRSox. Protein sulfhydryl loss was also partially restored after incubation with dithiothreitol. Sulfhydryl loss was not due to cell lysis. In addition, treatment of crude cellular particulate fractions with SIRSox resulted in protein sulfhydryl loss and formation of protein sulfenyl derivatives. A comparison of the amount of SIRS and H2O2 present to the number of protein sulfhydryls lost or sulfenyl derivatives formed suggests that SIRSox acts catalytically, serves as a co-factor in protein sulfhydryl oxidation, or that it activates a second pathway that is directly responsible for sulfhydryl oxidation.