Abstract: A calibration standard for calibrating machines which count the somatic cells in a milk sample, comprises an aqueous dispersion of microbeads bearing a fluorescent dye, a suspending agent, and an electrolyte; the dye has an excitation wavelength below 580 nm and a fluorescence emission wavelength in the range of 550 to 660 nm, with the excitation wavelength being at least 10 nm below the emission wavelength; the microbeads are present in a predetermined number per unit volume of dispersion; typically the standard contains 1.times.10.sup.5 to 9.times.10.sup.5 beads/ml.