We present evidence that nanotubular highways, or membrane nanotubes, facilitate a novel mechanism for intercellular communication in the immune system. Nanotubes were seen to connect multiple cells together and were readily formed between a variety of cell types, including human peripheral blood NK cells, macrophages, and EBV-transformed B cells. Nanotubes could be created upon disassembly of the immunological synapse, as cells move apart. Thus, nanotubular networks could be assembled from transient immunological synapses. Nanotubes were seen to contain GFP-tagged cell surface class I MHC protein expressed in one of the connected cells. Moreover, GPI-conjugated to GFP originating from one cell was transferred onto the surface of another at the connection with a nanotube. Thus, nanotubes can traffic cell surface proteins between immune cells over many tens of microns. Determining whether there are physiological functions for nanotubes is an intriguing new goal for cellular immunology.