A high speed ECL multiplexing unit which accepts 4 channels of data and control information is described. Compatibility with commercial circuits to facilitate economic interfacing at low speed data rates is an integral design criterion. Built from flip-flops and gates made in a 5 GHz fT chip technology which includes two-level metallisation and gold beam leads, the multiplexer is mounted on a ceramic substrate and contained in a standard DIL package. Dissipating less than 0.75 watt it outputs data faster than 300 Mb/s. Some conclusions concerning the merits of combining hybrid and monolithic technology are drawn.