1. The nuclei of cells from the thymus of the calf were isolated by three different techniques; the citric acid, the sucrose-calcium chloride, and the non-aqueous.
2. The mean dry weights of the nuclei were determined by chemical methods and by microscopic interferometry. There was a close correlation between the results from the interferometric and chemical methods.
3. The range of values about that mean was determined in each sample: the nuclei isolated in aqueous media contained approximately 45 per cent less material than those isolated in non-aqueous media.
4. The variations in dry weight with varying nuclear type are discussed.
5. The possible relationship between DNA content and dry weight is discussed.