T-cell colonies formed in liquid or semisolid cultures of stimulated human blood mononuclear cells were investigated under electron microscope. Typical gap junctions with five-layered structure were demonstrated between colony cells embedded in water miscible resin. The overall thickness of the junctional zones was less than 25 nm. The intercellular gap was demonstrated by lanthanum nitrate staining. Close membrane associations were observed after normal embedding with a junctional width of 20-35 nm corresponding to gap junctions. The functional importance of free communication between colony cells is discussed.