The binding ability of phytochrome to the liposome was checked in the presence and the absence of cyclic-AMP and estriol, respectively. The enhancement of the phytochrome binding (particularly Pfr form) to the liposome was observed in the presence of those compounds. However, the mode of enhancement was revealed to be different in the two cases. The phytochrome-c-AMP complex could even bind to the liposome prepared without cholesterol. At the stage of low concentration of liposome, no binding was occurred in this complex. On the other hand, the phytochrome-estriol complex could bind to the liposome from the initial stage of liposome addition. The difference absorption spectrum (Pr vs Pfr) of the phytochrome-c-AMP complex was essentially identical with the one of free phytochrome. But upon addition of liposome, the absorbance difference appeared to increase at the wavelength range of 600-630 nm. These results imply that 1) the binding site of c-AMP in phytochrome must be different with the one of estriol which is believed to be the same site of ANS and/or chromophore, 2) an electrostatic interaction is primarily involved in binding of phytochrome-c-AMP complex to the liposome in contrast with the cases of phytochrome-estriol and phytochrome-ANS complexes.