The endothelial adhesion molecules ICAM-1 and PECAM-1 internalize various particles, including natural ligands, apoptotic fragments and antibody conjugates used for drug delivery. The endocytic vesicles involved are 100-300-nm in diameter, which is compatible with both major endocytic mechanisms: the caveolar pathway and the clathrin-dependent pathway. Nevertheless the mode of ICAM-1/PECAM-1 internalization is not known. Michael Koval and co-workers have therefore analysed uptake of fluorescent anti-PECAM-1 and anti-ICAM-1 conjugates by human umbilical vein endothelial cells (see p. 1599). They find that neither conjugate colocalizes with clathrin or caveolin when internalized and that inhibitors of caveolae- or clathrin-mediated endocytosis fail to block their internalization. Moreover, the authors show that actin `cups' do not form around the internalized particles, eliminating the possibility that phagocytosis is responsible. Interestingly, however, they do find that ICAM-1-mediated endocytosis can be inhibited by a dominant negative mutant of dynamin 2 (a GTPase that helps to recruit actin to endocytosis sites) and by inhibitors of protein kinase C (PKC), Src or Rho kinase (ROCK). They have thus identified a novel endocytic mechanism as well as several of the signalling molecules involved.