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A continuous precipitation and filtration flow system coupled on-line with an atomic absorption spectrometer for the quantitative pre-concentration and determination of lead at the micrograms per litre level is described. PbII forms a precipitate with ammonia, which is retained on a stainless-steel filter and then dissolved by a stream of nitric acid. A concentration factor of up to 700 is achieved. Several calibration graphs suited to the degree of enrichment are proposed for the determination of lead(II) in the range 1.2–1500 ng ml–1; sampling rates of 1–15 h–1 and relative standard deviations of less than 3.6% are afforded by the proposed method, which has been satisfactorily applied to the determination of lead in tap water.