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Four proteins of the lipocortin family, lipocortin I (35 kDa), lipocortin II (36 kDa), lipocortin V (32 kDa) and lipocortin VI (67–70 kDa), were identified in the cytosols of 2-day-old cultures of thyroid cells. Only lipocortin I was phosphorylated in vitro in fully differentiated, thyroid stimulating hormone-treated cells (0.1 mUml). Protein kinase C was the only kinase activity which phosphorylated lipocortin I. Phosphorylation shifted its p Ifrom 6.9 to 6.6. The in vitro phosphorylation of lipocortin I was impaired in cultures exposed for 2 days to phorbol ester (10 −7M), although it was present in both the cytosol and the particulate fraction of these cells.