A Ca 2 + - and phospholipid-dependent protein kinase (CaPK) has been identified in rat liver microsomes. CaPK isolated from liver cytosol bound to smooth microsomes in the presence of 100 μM CaCl 2 . A saturation in binding was observed when a 5-fold excess of enzyme over that present in microsomes had become bound. The microsomal CaPK and 50% of the enzyme bound in vitro was not removed by EGTA treatment. This suggests that Ca 2 + is required for the binding of CaPK to microsomes, but not for the retention of the enzyme on the membrane.