Phosphoinositide (PI) 3‐kinases have been characterized as enzymes involved in receptor signal transduction in mammalian cells and in a complex which mediates protein trafficking in yeast. PI 3‐kinases linked to receptors with intrinsic or associated tyrosine kinase activity are heterodimeric proteins, consisting of p85 adaptor and p110 catalytic subunits, which can generate the 3‐phosphorylated forms of phosphatidylinositol (PtdIns), PtdIns4P and PtdIns(4,5)P2 as potential second messengers. Yeast Vps34p kinase, however, has a substrate specificity restricted to PtdIns and is a PtdIns 3‐kinase. Here the molecular characterization of a new human PtdIns 3‐kinase with extensive sequence homology to Vps34p is described. PtdIns 3‐kinase does not associate with p85 and phosphorylates PtdIns, but not PtdIns4P or PtdIns(4,5)P2. In vivo PtdIns 3‐kinase is in a complex with a cellular protein of 150 kDa, as detected by immunoprecipitation from human cells. Protein sequence analysis and cDNA cloning show that this 150 kDa protein is highly homologous to Vps15p, a 160 kDa protein serine/threonine kinase associated with yeast Vps34p. These results suggest that the major components of the yeast Vps intracellular trafficking complex are conserved in humans.