Peripheral blood mononuclear cells (MNC) of rats, which had received a subcutaneous inoculation of either viable or X-irradiated syngeneic DMH-W49 colon carcinoma cells, were investigated sequentially for sensitization against tumor-associated antigens by a new micro-glass-tube leukocyte adherence inhibition (LAI) assay. The number of adherent cells of MNC subpopulations was estimated by a cellular radioimmunoassay (CRIA), which utilizes anti-monocyte (MC) antiserum or anti-T-cell monoclonal antibody (McAb) and 125I-labelled protein A. LAI reactivity was demonstrated in rats sensitized with X-irradiated tumor cells when assessing the adherence of both T lymphocytes and monocytes. It was found that reactivity with the two different types of indicator cells did not usually coincide in time. The same phenomenon was also observed in tumor-bearing rats although with lower levels of LAI reactivity. In both groups the LAI response detected by anti-T-cell reagent mostly appeared earlier after tumor-cell inoculation than the response demonstrated by anti-MC serum. The T-lymphocyte-associated reactivity also disappeared more rapidly. These results suggest that different states of immune reactivity are reflected in LAI responses involving T lymphocytes and monocytes as indicator cells.