The humoral and cellular immune status of C57BL/6 male mice and "R" male rats bearing MC-induced sarcomas were investigated in vitro, using 51Cr-releasing and mixed hemadsorption assays. Analysis was performed on mice subjected to concomitant tumor immunity (CTI) model experiments or bearing primary tumors of different sizes, that was done also in rats. Chromatographic fractions, assumed to contain tumor specific antigens, antibodies or antigen-antibody complexes were identified by their absorption capacity upon specially prepared syngeneic immune sera. Free antibody and antigen-antibody complexes, accompanied with a weak cell mediated immunity (CMI) were correlated with efficient CTI in 2--3% TW/TBW tumor bearers. Homoral immunity (HI) showed sometimes an important increase after challenge administration. High levels of free antigen and antigen-antibody complexes, lack of antibodies and CMI were correlated with CTI absence in huge tumor bearers, representing 30--37% of total body weight. Conditions determining variability of results reported by different authors and the possible mechanisms by which serum immune factor may impair the tumor bearer's immune status are discussed.