The California Mastitis Test (CMT) is a test used for estimating the number of somatic cells in a milk sample. A reagent (secondary alkyl sulphate) is added to the milk and the number of somatic cells estimated from the resultant viscosity of the mixture. The Ruakura rolling ball viscometer has been developed to measure the viscosity of the CMT gel. It consists of a tube mounted horizontally so that the milk-reagent mixture can be introduced into it. After a holding time of 26 seconds the tube tips through an angle of 25° allowing a stainless steel ball to roll through the mixture for a standard time before returning to the horizontal. The distance the ball has rolled is inversely related to the viscosity developed. The physical function of the viscometer follows Stokes' Law. The range of the instrument is suited to reading the viscosity developed by the CMT reaction in New Zealand herd milks. The viscous reaction in milk is caused by somatic cell nuclei forming a fibrillar network. The reaction intensity is related to the DNA content of the milk sample. Proteins can modify the viscosity developed. Bacterial DNA does not enter the reaction. The network is fragile and rheomalaxic in nature. The reaction intensity in milk varies with time. An approximate 25 second holding time is required before determining the viscosity. After this holding time the reaction intensity progressively decreases. Milk to be tested should be under 24 hours old although refrigeration or boric acid preservative can extend this time. Different operators can rank milk samples by somatic cell count in a similar order with the Ruakura rolling ball viscometer. There is a significant correlation (r = 0.92***) between Ruakura rolling ball viscometer and somatic cell count of herd milk samples.