There have been given the results of the spectrophotometric examination of the dogs' and rats' venous and whole blood plasma taken in the process of the peritonitis progressing within the spectral interval 320 220 − = λ nm (for plasma) and 610 350 − = λ nm (for the whole blood). It has been defined that D-optical density values in the field of the longwaved maximum of plasma absorption intensity of the venous blood at 280 = λ nm depend upon the intensity of the inflammatory process and also upon the circumstances against the background of which it started to progress. It was found out that the dynamics of D-values changes for 540 = λ (or 570) nm in the process of the peritonitis progressing in case of the whole blood taken from a portal vein is a mirror symmetrical if to compare to the same dynamics for the blood from cava inferior. The defined conformities with regularities may have a diagnostic meaning. It was also found out that the biggest influence upon the dynamics of D-values at λ = 280nm of the venous blood plasma has the content of the circulating immune complexes, necrosis factor of α -tumors and interleukin -2, the changes of which explain for almost on 100% the distribution of the optical density parameters and what proves a possible immunologic explanation of its changes.