Th1 and Th2 subsets have been characterized on the basis of the cytokines they secrete and the immune functions they mediate. Th1 cells secrete IL-2, IFN-gamma, and lymphotoxin and are important in the cell-mediated response; Th2 cells secrete IL-4, IL-5, IL-10, and IL-13 and are important in the control of macrophage function and in the stimulation of particular immunoglobulin isotypes. Cytokines secreted by Th1 and Th2 cells regulate the growth and differentiation of Th1 and Th2 cells in both positive and negative ways; this has been termed crossregulation. Much work has concentrated on the factors important in the differentiation of these Th subsets, and it has been established that Th cells become committed to a Th1 or Th2 phenotype within 48 hrs of antigenic stimulation. During the differentiation process irreversible changes in the expression and function of cytokine receptors occur that provide an explanation for the observed crossregulatory features of Th1 and Th2 cells. In this review we summarize the crossregulation between Th1 and Th2 cells in terms of the changes in cytokine receptor expression and function that occur during differentiation.