The antibody prevalence rate of 3 types of IMV in 24 sera of Japanese patients with colorectal adenocarcinoma (mean age 60.8 years) was 83.3% (20/24). Of 20 positive sera, 19 contained type 1 antibody and one contained type 2. In contrast, the seropositivity of IMV in 21 sera of matched control patients (mean age 60.9 years) with non-colorectal carcinoma such as mammary and lung carcinoma was 23.8% (5/21). The 5 positive sera contained type 1 antibody. The seropositivity to IMV in 79 sera of healthy Japanese over 50 years of age (mean age 62.3 years) was 24.0% (19/79). Again all of the 19 positive-sera contained type 1 antibody. The seropositivity of IMV in the patients with colorectal adenocarcinoma was significantly higher than in the controls, non-colorectal carcinoma patients and healthy persons over 50 years of age (P less than 0.001). Type 1 IMV was isolated from each of 2 recently established Japanese colorectal adenocarcinoma cell lines.