A small flow-cell, consisting of a fused silica capillary (200 mum diameter), was constructed for use in fluorimetry. A near-infrared semiconductor laser was used as an excitation source, and an optical fibre (core diameter 50 mu m) inserted into the capillary tube was used as a waveguide for light introduction or fluorescence collection. The volume of the flow-cell was 3-60 nl. A polymethine dye could be detected in the range 12-90 fg, and the absolute amount of the sample in the detection volume was 140-370 ag.