A large area proportional counter designed for the determination of 239Pu in human lungs at low levels is described. The counter uses the multiwire method of internal anticoincidence for background reduction. In addition, the technique of pulse rise-time discrimination in favour of low energy X-rays is applied. The background of the counter (window area 900 cm2) is 2.2 counts/min in the energy band from 10 keV to 24 keV used for 239Pu determinations. The counter has been calibrated at 20 keV using seven volunteer subjects who inhaled aerosols labelled with 17 day 103Pd. The derived minimum detectable activity for a subject of average build is 8 nCi of 239Pu, i.e. about half the maximum permissible lung burden.