Binding of bilirubin with particles of synaptosomal membrane (pH 7.2, 20 degrees) resulted in shifting of absorption maximum of bilirubin aqueous solution towards the long-wave region with maximal extinction at 460 nm and the plateau at 480-510 nm as well as in amplification of the pigment fluorescence at 530-540 nm (excitation at 430 nm). Two types of bilirubin binding sites were detected in synaptosomal membrane. At the sites of the first type content of bound bilirubin (calculated per membrane proteins) constituted 5 nmol/mg (Ka = 0.6.10(5) M-1). This value was 7 nmol/mg (Ka = 0.2.10(5) M-1) at the sites of the second type. Alterations in pH of a medium within ranges from pH 7.2 to pH 8.0 led to reverse transition of the bilirubin free form (pH 8.0) to the form bound with membrane particles (pH 7.2). On the molar ratio of 1:30 of human blood albumin and the bilirubin, bound with plasmatic membrane, shifting of pH within the ranges of pH 7.0-7.6 was followed by reverse binding of bilirubin with synaptosomal membranes (pH 7.2) or with albumin (pH 7.6).