Investigations have been made on the minimisation of the background count rate detected in proportional counters, while maximising the signal detection efficiency. Two methods of background rejection have been developed, based on the time profile of the shaped detector pulses and on the lateral extent of the induced charge distribution on the cathode. Both these systems have proved highly efficient, and the former has been applied to a multiwire proportional chamber designed as a monitor for plutonium lung contamination, an application where high efficiency detection is critical. This instrument, which incorporates full six-sided "massless" guard cells, and utilises real-time data acquisition and handling by microcomputer, has been developed, optimised, and assessed as a possible competitor to the phoswich detectors in use at present and the solid state detectors under development. Results indicate that the counter is likely to offer no more than a marginal improvement in sensitivity, insufficient to justify its increased compexity and higher costs.