1. The particulate form of lactating bovine mammary lactose synthetase activity is shown to be more highly organized than previously reported. 2. A novel method of shattering frozen mammary tissue with effective cell disruption is described. 3. The apparent subcellular distribution of lactose synthetase was shown to reflect the method of homogenization. 4. After mild homogenization particles associated with a high content of intact lactose synthetase activity sedimented in the lysosome size range between 5x10(4) and 3x10(5)g-min. 5. Lactose synthetase was dissociated and solubilized by VirTis homogenization and ultrasonic treatment. The activities and behaviour of UDP-galactose hydrolase, succinate dehydrogenase, beta-glucuronidase and phosphodiesterase I were compared. 6. Inhibition of UDP-galactose hydrolase by UTP and alpha-lactalbumin was observed.