Abstract Patterns of protein phosphorylation were examined in particulate fractions prepared from normal breast from 111 women. Material was obtained from a variety of physiological states including pregnancy (16 cases), lactation (4 cases), post-lactational involution (6 cases), prolonged involution (11 cases) and the ‘resting’ state (74 cases; 52 nulliparous and 22 parous). Total level of phosphorylation varied with the physiological state of the tissue, being higher in pregnancy, lactation and post-lactational involution than in the resting breast. The molecular weights of major bands phosphorylated were 180, 52, 48 and 44 kDa. Evidence from base hydrolysis in five cases, indicated that the 180 kDa band was phosphorylated primarily on tyrosine residues. Quantitation of this protein showed variability within sub-groups, but levels were highest in post-lactational involuting breast. Sub-division of the nulliparous group according to phase of the menstrual cycle, oral contraceptive use and breast epithelial proliferation, showed significantly lower phosphorylation in this band amongst oral contraceptive users. In contrast to the 180 kDa band, base hydrolysis of the protein at 44 kDa did not indicate phosphorylation on tyrosine residues. Levels of phosphorylation were highest in pregnancy and lactation and were significantly reduced in examples showing prolonged involution. In the nulliparous resting breast, phosphorylation of the 44 kDa band was not associated with phase of the menstrual cycle or oral contraceptive use. However, there was a strong positive association between the phosphorylation of 44 kDa band and level of proliferation.