The somatic cell count of milk is routinely determined by the fluoro-opto-electron method and sometimes by the direct epifluorescent filter technique (DEFT). This paper investigates the potential of solid phase cytometry (SPC), a novel technique combining aspects of both the fluoro-opto-electronic method and epifluorescence microscopy for somatic cell counting. In SPC, cells are retained on a membrane filter, fluorescently labelled and automatically detected on the entire membrane filter by means of a laser scanning instrument ChemScan). Fluorescent spots can be visually inspected by an epifluorescence microscope with a computer-driven moving stage. The performance of SPC was compared with that of the fluoro-opto-electronic method using a Fossomatic 360 instrument for 68 milk samples with varying somatic cell counts (10(3)-10(6)/ml). The sample throughput and repeatability of SPC were inferior to those of' the Fossomatic method and statistical analysis of the method comparison data using the approach of J. M. Bland & D. G. Altman (The Lancet 1986 February 8 pp 307-310) revealed a poor comparability between the two methods. Moreover, problems of milk filterability and the interference of fluorescent particles presently hamper the routine application of SPC. Nevertheless, this method represents the first example of the application of SPC to milk.