Insulin imprinting of Tetrahymena pyriformis in different growth phases had been investigated. Cells formed in the early logarithmic phase (18-hour culture) showed enhanced hormone binding at the second encounter with the hormone proving that imprinting had developed. This phenomenon was not observed in cells formed in the late logarithmic phase (42-hour culture) or in the stationary phase (66-hour culture). Lipid transformation processes, alteration of the guanyl cyclase activity and enhanced cell division may be responsible for this effect. Cell-growth phase G1 was especially favourable for development of imprinting.