The lymphocyte activation gene (LAG) -3 is a member of the immunoglobulin super-family that is selectively transcribed in human activated T and NK cells. In this work, the possibility that LAG-3 expression by human CD4+ T cells was preferentially related to one or another phenotype of cytokine secretion was investigated. Surface LAG-3 expression correlated with IFN-gamma, but not IL-4, production in antigen-stimulated T cells and it was up-regulated by IL-12. Most activated CD4+ T cell clones with established Th1 or Th0 profiles of cytokine secretion expressed LAG-3 on their surface, whereas the great majority of Th2 clones showed neither surface LAG-3 nor LAG-3 mRNA expression. After activation, the majority of CD4+ T cell clones also released soluble LAG-3-related peptides, and such a release correlated positively with the production of IFN-gamma and inversely with the production of IL-4. Thus, LAG-3 expression by activated CD4+ human T cells appear to be preferentially associated with the differentiation/activation pathway leading to the production of IFN-gamma.