Milk from a number of species (e.g., man, mouse, rat, dog, and cow) contains inhibitors of the RNA-directed DNA polymerase. When attempts are made to isolate virions from the milk, part of the inhibitors follow the virions in the purification. The amount of inhibitors varies in different milk samples. These inhibitors can probably account for the large discrepancies reported in studies of the presence of oncornaviruses in human milk. Phosphatases bound to subcellular particles or fragments seem to be the most important inhibitors in the milk interfering with the RNA-directed DNA polymerase assay. It is shown that the inhibitory enzymes can be completely removed by sedimentation of the milk through a Metrizamide gradient.