A laboratory experiment is presented in which the freezing point depression is analyzed using milk as solution. The nature of milk as a mixture of different solutes makes it a suitable probe to learn about colligative properties. The first part of the experiment illustrates the analytical use of freezing point measurements to control milk quality, regarding adulteration by addition of water and lactic fermentation. The second part deals with molecular weight determination from freezing point depression data, using powdered milk as solute. The effective molar masses of whole milk and skim milk are determined and compared. (Contains 2 tables and 2 figures.)