Suppressor T cells modulate both humoral and cell-mediated immune responses by antigen-specific and nonspecific mechanisms. Moreover, soluble factors either secreted by or extracted from these suppressor T cells efficiently mediate the immunoregulatory activities of these cells. The molecular properties and mechanism(s) of action of a nonspecific and an antigen-specific suppressor T-cell factor, both of which regulate antibody responses, will be compared and contrasted. The nonspecific factor, soluble immune response suppressor (SIRS), is produced by Concanavalin A-stimulated T cells and is not separable from MIF activity. The antigen-specific suppressor factor (GAT-TsF) is extracted from T cells stimulated with the synthetic terpolyper L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT). These two factors differ markedly in molecular properties, target cells and mechanism(s) of action. The comparison of the mechanism(s) of action of nonspecific and antigen-specific suppressor T-cell factors provides useful insights into the various pathways operative for regulation of antibody responses. The understanding of the interrelationships among these two classes of immunoregulatory molecules and how they may act to modulate antibody responses are essential for therapeutic manipulation of immune responses and control of the inflammatory response. This is especially important with the nonspecific factor, SIRS, and provides a model in which to study the interface of immune responses and inflammatory responses.