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As the application of flow cytometry increases across the environmental sciences, there is a need to expand instrument flexibility. In particular, some of the major applications within aquatic research demand an ability to quantify the particles under investigation, expressing them as numbers per unit of sample volume. Two of the principal areas of flow cytometric investigations are the characterisation of naturally occurring particulate assemblages in the oceans and investigations into the feeding behaviour of “filter feeding” organisms on the mixtures of such particles. In both cases some of the most useful data derived from flow cytometry require that analyses be determined in terms of particle concentrations. Therefore, a simple and practical modification is described, which allows known sample volumes to be analysed on flow cytometers. The method has a high degree of flexibility in sample volume and is cost effective and reliable. It involves introducing a standard chromatography “loop injection valve” into the sample-handling fluidics system. Use of fluorescent beads shows that a range of praticle sizes and concentrations can be used with this system. Tests, also using fluorescent beads and an algal culture, show that replicate 100 μ1 sample volumes can be analysed and counted with a coefficient of variation of less than 3%. © 1994 Wiley-Liss, Inc.