En masse secretion of milk proteins, notably the caseins in the form of casein micelles, is a unique feature of the milk-secreting mammary epithelial cell. Caseins are therefore specific markers of these cells and constitute an ideal tool to monitor their differentiation, as well as functional status, during the development of the gland. To use them as such, a reliable method for quantitative analysis of the caseins from mammary cells or tissue is needed. Here we show that the caseins are heat-stable, a feature that leads to their complete extraction from a complex cellular extract by boiling. This allows for high enrichment and direct analysis of the caseins, even when they are poorly expressed in the starting material.