A body 300 having a cavity 310 for mounting a substrate 120 fabricated with probe sequences at known locations according to the methods disclosed in U.S. Pat. No. 5,143,854 and PCT WO 92/10092 or others, is provided. The cavity includes inlets 350 and 360 for introducing selected fluids into the cavity to contact the probes. Accordingly, a commercially feasible device for use in high throughput assay systems is provided.