Cheddar cheese was made from milks with and without growth of psychrotrophic bacteria before pasteurization. Chemical and bacteriological evaluations were made on the milks and on whey and cheese from the milks. Sensory evaluation was also done on cheese samples. The percentage of nonprotein nitrogen and noncasein nitrogen increased over control milks when milk with psychrotrophic bacteria was held at 4.4C for 7 days. Processing time was decreased and a firmer curd resulted when Cheddar cheese was made from milk cultured with psychrotrophic bacteria rather than control milk. Differences were minor for milkfat, moisture, nitrogen content, and pH between cheeses made from control and precultured milks. Sensory evaluation revealed little difference between cheeses from control and treated milks at 3 mo of ripening at 4.4 or 10C. Unacceptable changes became evident at 6 mo, especially for cheeses ripened at 10 C.