The staphylococcal enterotoxins (SEs) are homologous proteins related in their capacity for stimulating both T cells and monocytes. To assess the importance of conserved structure and sequence to functional activity, the role of the disulfide loop and adjacent sequence in these toxins was evaluated. Contrary to previous reports, we demonstrate here that the disulfide loop was required for the mitogenic activity of SEA and SEB. While T cell-stimulatory activity was compromised, reduced and alkylated SEs retained major histocompatibility complex class II-binding and monocyte-stimulatory activities, suggesting that their inability to induce T cell proliferation was due to failure to interact with T cell receptor (TCR) rather than with class II molecules. Reduction and alkylation did not affect the far-ultraviolet circular dichroic spectrum of SEA, suggesting that the loss of mitogenic activity was not associated with significant changes in secondary structure. The disulfide linkage imparts considerable stability to these toxins as peptide cleavages within the loop of SEB were not associated with detectable loss of function, although cleavage in the conserved sequence outside the loop of SEA resulted in loss of mitogenic activity. This report thus establishes a functional role for a conserved element in SEs, the disulfide loop, and further indicates that their class II- and TCR-binding activities can be dissociated. © 1990, Rockefeller University Press., All rights reserved.