An intercomparison study of low-level 237Np determination in artificial urine samples has been carried out. The purpose of this study was to find the "optimal" method presently available for use in a routine in-vitro radiobioassay program for occupationally exposed workers. Four synthetic urine samples with differing 237Np concentrations were prepared: (1) 3 mBq kg-1 of 237Np; (2) 3 mBq kg-1 of 237Np with natural uranium, 239Pu and 241Am as interferences; (3) 50 mBq kg-1 of 237Np; and (4) a matrix blank. The solutions were submitted to 10 alpha-particle and 10 inductively coupled plasma-mass spectrometry (ICP-MS) laboratories of which six and four laboratories, respectively, returned results. Two laboratories performed assays using both techniques. The radiochemical method of choice used 239Np as the tracer, which utilized a combination of coprecipitation and anion-exchange separation. The best results obtained by ICP-MS were comparable with but not better than the most accurate results obtained by alpha-particle spectrometry. Alpha-particle spectrometry measurements overall gave consistently better agreement with known values.