The UV absorption and resonance Raman spectra were recorded from the trimer allo-phycocyanin and pH-induced monomer allophycocyanin from blue-green alga Phormidium Iuridum.Monomerization mostly results in the disappearance of characteristic absorption peak 650nm,and in the changes in relative intensities and positions of Raman bands of phycocyanobilin chromophores.As a result,in the monomeric allophycocyanin,the chromophores adopt cyclohelical conformations,which close to these of free chromophores in vitro; and in the trimeric allophycocyanin, fully extended conformations of the chromophores can be marked by the main Raman peak around 1645 cm^-1, the behaviour of which is in Rood agreement with the A_vis/A_uv, absorption ratio.