The regulatory and catalytic properties of the three mammalian isoforms of protein kinase B (PKB) have been compared. All three isoforms (PKBα, PKBβ and PKBγ) were phosphorylated at similar rates and activated to similar extents by 3-phosphoinositide-dependent protein kinase-1 (PDK1). Phosphorylation and activation of each enzyme required the presence of PtdIns(3,4,5)P₃ or PtdIns(3,4)P₂, as well as PDK1. The activation of PKBβ and PKBγ by PDK1 was accompanied by the phosphorylation of the residues equivalent to Thr³⁰⁸ in PKBα, namely Thr³⁰⁹ (PKBβ) and Thr³⁰⁵ (PKBγ). PKBγ which had been activated by PDK1 possessed a substrate specificity identical with that of PKBα and PKBβ towards a range of peptides. The activation of PKBγ and its phosphorylation at Thr³⁰⁵ was triggered by insulin-like growth factor-1 in 293 cells. Stimulation of rat adipocytes or rat hepatocytes with insulin induced the activation of PKBα and PKBβ with similar kinetics. After stimulation of adipocytes, the activity of PKBβ was twice that of PKBα, but in hepatocytes PKBα activity was four-fold higher than PKBβ. Insulin induced the activation of PKBα in rat skeletal muscle in vivo, with little activation of PKBβ. Insulin did not induce PKBγ activity in adipocytes, hepatocytes or skeletal muscle, but PKBγ was the major isoform activated by insulin in rat L6 myotubes (a skeletal-muscle cell line).