Abstract: A heat exchanger core is formed by a plurality of stacked plates which are rectangular so as to define two fluid paths at right angles passing between alternate ones of the plates. The core is constructed by defining walls extending at right angles from the side edges of one of the plates and an inturned flange at the extremity of each wall. The inturned flange cooperates with a slot defined by a pair of folded portions of the next adjacent sheet so that the core is assembled by sliding two opposed slots along the cooperating flanges of the next adjacent sheet. A corner member has projections from the edges of a pair of right angle strips which projections extend into the core alternately into one fluid path and the next adjacent fluid path. Side support members similarly include a flat strip and projections into the fluid paths of the core.