Using the method of flow cytofluorometry we found that proteinase activity eliminating antigenic determinants from the surface of tumor cells disappeared from the serum of mice with Ehrlich carcinoma. This activity towards Ehrlich carcinoma cells is present in the sera of mice without tumors and in mice with other transplanted tumors. The serum from mice of one strain with Ehrlich carcinoma showed no protease activity against Ehrlich carcinoma cells in mice of other strain. Hence, Ehrlich carcinoma growth is associated with tumor-specific changes in the serum resulting in disappearance of specific protease activity of the serum against tumor cells.

Experiments on female (CBAxC57Bl/6)F1 mice with simultaneously transplanted Ehrlich carcinoma and B16 melanoma showed that removal of one of the primary nodes led to metastasizing of the removed tumor alone. It seems that this specificity of the inhibitory effect of the primary tumor can be explained by peculiarities of glycosylation and subsequent complex formation of serum proteins with the tumor.

In male C57Bl/6 mice with transplanted Ehrlich carcinoma, hemoglobin forms a complex with serum proteins characterized by a molecular weight of about 300 kDa. The complex incorporates proteins weighing 100, 68, 65, and 15 kDa identified by MALDI-TOF mass spectrometry as haptoglobin, serum albumin, gi/26341396 nameless protein Mus musculus, and alpha-hemoglobin, respectively. This complex can possess biological activity and contribute to the control of tumor growth.