Mammary epithelial cells from lactating mice were used to compare the in vitro casein synthesis by membrane-bound and free polysomes with that of messenger ribonucleic acid extracted from free- and bound polysomes (polysomal system and a rabbit reticulocyte lysate system). Casein synthesis was detected by incorporation of hydrogen-3 labeled amino acids in the reaction mixture into synthesized casein molecules. Membrane-bound polysomes synthesized considerably more casein than free polysomes; this difference was abolished by deoxycholate treatment of free polysomes. The synthesizing activity of messenger ribonucleic acid was similar whether extracted from free- or membrane-bound polysomes. Casein synthesis may be controlled by factors on the polysomal membrane.

Rat mammary glands contain cyclic AMP-independent casein kinase and cyclic AMP-dependent histone kinase. The former was easily isolated from cyclic AMPdependent histone kinase by MgCl2 treatment.Mammary casein kinase was not activated by cyclic nucleotides, and Mg++ and ATP were required for activation.The specific activity of casein kinase in cytosol of rat mammary epithelial cells increased 2 to 3-fold during pregnancy and lactation.Cytosol of mouse mammary epithelial cells also contained cyclic AMP-independent casein kinase, and the activity of this enzyme was about three times that of the Golgi fraction.

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