A rapid method for the direct enumeration of somatic cells in fresh and formalin-preserved raw milk is described. Heat treatment at 80°C for 10 mn fixed somatic cells sufficiently to prevent lysis and subsequent dilution allowed the equivalent of at least 0.5 ml milk to be filtered through a 1.0 μm pore size Nuclepore membrane filter. The somatic cells were concentrated on the membrane and, after staining with acridine orange, fluoresced green, yellow or orange under an epifluorescent microscope. Three different types of cell could be distinguished on the basis of nuclear structure and cell size. These were monocytes, polymorphonuclear leucocytes and the larger epithelial and/or secretory cells. For both fresh and preserved milks the count of somatic cells on the membrane correlated well (r ≽ 0·93) with the Coulter count. Differences between counts obtained by different operators were not significant. The technique is rapid, taking about 20min, and is suitable for milks containing between 2 × 104 and 1 × 107 somatic cells/ml.