A simple enrichment method for trace amounts of Cu(II), Fe(III), Pb(II), Co(II) and Cr(III) has been established. The preconcentration/separation procedure is based on chelate formation of the analyte metals with benzopurpurine 4B and on retention of the chelates on cellulose nitrate membrane filter. After completely dissolving the cellulose nitrate membrane and analyte ions by 500 muL of nitric acid at 85degreesC on a hood, metal determinations were performed by flame atomic absorption spectrometry. The influences of some analytical parameters including pH, types of membrane filters, amounts of reagent etc. on the recoveries of copper, iron, lead, cobalt and chromium were investigated. The detection limits of the analyte ions (k = 3, N = 2 1) were varying 1.9 mug/L for cobalt to 10.5 mug/L for iron. The method was applied to natural water samples for the determination of analyte ions with satisfactory results (recoveries > 95%, RSD < 10%). The relative standard deviations (RSD) of the determinations for analyte ions were below 10%.