The aim of this work was to test whether the nature of the antigen-presenting cell influences the Th1/Th2 balance in vivo. Adoptive transfer of dendritic cells or macrophages, pulsed extracorporeally with antigen, results in antigen-specific T cell priming. Of note, macrophages and dendritic cells appear to differentially regulate the development of T lymphocytes. In addition, the population of splenic dendritic cells appears heterogeneous and includes the CD8alpha+ and CD8alpha- subclasses which direct the differentiation of Th1 and Th2 cells, respectively. Using neutralizing antibodies and mice genetically deficient for various cytokines, we evaluated the role of several molecules in the development of T cells in vivo. Our observations emphasize the role of CD86 and IL-6 for Th2 priming by macrophages, CD86 for Th1 priming by dendritic cells and IL-12 for Th1 priming by dendritic cells.