A functioning agglutination mechanism is paramount to the quality of non-homogenized milks, because the process of agglutination causes firm cream layers in bovine milk. Agglutination is believed to occur due to interactions between the surface of milk fat globules and immunoglobulins, yet direct observations of the phenomenon have until now not been achieved. The presented research demonstrates how the process of agglutination can be visualized using confocal laser scanning microscopy, rhodamine red and a fluorescent immunoglobulin M (IgM) antibody. The effect on agglutination of storage temperature and pasteurization temperature was studies. Storage at 5°C resulted in clearly detectable agglutination, which was reduced at 15°C. Increasing storage temperature to 20 or 37°C cancelled any detectable interaction between milk fat globules and IgM. Thus the existence of cold agglutination was documented. Progressively higher inactivation of IgM and, hence, reduction of agglutination was observed, when pasteurisation (20 s) was increased from 69°C to 71°C and further to 73°C. Furthermore, 2-dimensional gel electrophoresis revealed a redistribution of Ig-related proteins in milk fat globule membrane isolates in response to changes in storage temperature. Poly-immunoglobulin G receptor was detected in milk fat globule preparations stored at cold (4°C) conditions, but was absent after storage at higher temperature (25°C). The findings provide valuable information about the right pasteurization temperature for dairy producers of non-homogenized milk.