Activity of endothelial Tie2 receptor tyrosine kinase is modulated by two naturally occurring, secreted ligands, angiopoietin-1 and -2, which have opposing effects on its phosphorylation. Receptor tyrosine kinase activation requires receptor dimerization/multimerization, which, for many receptors, is mediated by homo-oligomeric ligands binding to and bridging receptor molecules. We show here that angiopoietin-1 and -2 form distinct arrays of disulfide-linked homo-oligomeric complexes. Their mobilities on nonreducing gels suggest that angiopoietin-2 exists predominantly as a homodimer but also forms higher order multimers. In contrast, angiopoietin-1 forms some homotrimers, but predominantly exists in higher order multimers. These two structurally related, 60% homologous ligands are predominantly composed of an amino-terminal coiled coil domain and a carboxyl-terminal fibrinogen-like domain. We show that their distinct oligomerization patterns are determined by their coiled coil domains and, furthermore, that their coiled coil domains, but not their fibrinogen-like domains, are sufficient to mediate formation of disulfide-linked homo-oligomers. In contrast, the differential effects of these ligands on endothelial Tie2 phosphorylation is mediated by their fibrinogen-like domains. We conclude from these studies that the coiled coil and fibrinogen-like domains of the angiopoietins have distinct functions with the coiled coil domain mediating ligand homo-oligomerization and the fibrinogen-like domain mediating ligand activity.