Nonfat (0% fat), reduced-fat (11% fat), and control (19% fat) mozzarella cheeses were made using direct acidification to test the influence of three levels (0.25X, 1X, and 4X) of coagulant concentration on proteolysis, meltability and rheological properties of cheeses during 60 d of storage at 5°C. Changes in meltability, level of intact αs1-casein and β-casein (by capillary electrophoresis), 12.5% TCA-soluble nitrogen, and complex modulus were measured. There were differences in rate of proteolysis and functional properties as a function of fat content of the cheese, but some of these differences could be attributed to differences in moisture contents of the cheeses. As fat level decreased, the percent moisture-in-nonfat-substance of the cheeses also decreased. Cheeses with the lower fat contents (and consequently the lowest moisture-in-nonfat-substance content) had slower rates of proteolysis. Fat content influenced the complex modulus of the cheese, with the biggest effect occurring when fat content was reduced from 11 to 0%. Coagulant level had only a small effect on initial modulus. Cheeses became softer during storage, and the decrease in modulus was influenced by the level of coagulant. At 0.25X, there was very little decrease in modulus after 60 d, while at 1X and 4X coagulant levels the softening of the cheese was more evident. The influence of coagulant level and fat content on cheese melting was similar to their effects on complex modulus. In general, higher fat contents promoted more melting and so did higher coagulant levels. Melting increased during storage although very little change was observed in the nonfat cheese.