A new, highly sensitive micro-glass-tube leukocyte adherence inhibition (LAI) assay has been developed to detect anti-tumor sensitization in a rat colon carcinoma system. This technique requires fewer cells and smaller amounts of antigen preparations than the previous glass tube method. The microscopic enumeration of adherent cells is replaced by a cellular radioimmunoassay (CRIA) which utilizes antibodies binding to mononuclear cells (MNC) and 125I-labelled protein A. Antigen preparations were shown to be adsorbed to glass. Precoating of glass vials with fetal calf serum or antigen preparations was shown to cause a major increase in nonspecific LAI. The new LAI technique is designed to minimize such nonspecific LAI. By applying an anti-T-cell monoclonal antibody (McAb), an anti-Ig antiserum and an anti-monocyte (MC) antiserum it became possible to assess selectively the change of adherence of T cells, B cells and monocytes. The specificity of the assay was demonstrated with anti-T-cell and anti-MC reagents in criss-cross experiments using two defined antigens and different tissue extracts as test antigens. The LAI response of peripheral blood MNC was examined after the subcutaneous isografting of 1 X 10(7) X-irradiated DMH-W49 colon carcinoma cells. Six days after sensitization all the rats showed an LAI response detectable with anti-T-cell and anti-MC reagents, which gave considerably higher LAI indices than anti-MNC antiserum. After the subcutaneous isografting of 1 X 10(5) viable DMH-W49 cells, 12 of 24 rats (50%) showed an LAI response detectable with the anti-MNC antiserum during a 3-week follow-up period. On the other hand, an LAI response was detected in all the 14 tumor-inoculated rats tested when the adherence of T lymphocytes and monocytes was assessed selectively.