Sixteen patients with colorectal carcinoma Dukes' Stage B2, C, or D were treated with an autologous virus-modified tumor-cell vaccine after potential curative tumor resection (R0-Resection). An inoculum of 1 X 10(7) cells incubated with 32 hemagglutination units of nonirradiated Newcastle disease virus (NDV) was given intracutaneously up to four times at 10-day intervals. The delayed-type hypersensitivity (DTH) skin reaction was measured. The vaccination was well tolerated. In 11 of 16 patients an increasing reactivity against the vaccine was observed during the vaccination procedure. A challenge test using autologous tumor cells without NDV after the vaccination cycle revealed a specific antitumor sensibilization in 12 patients. The DTH response was not due to bacterial contamination or sensibility to the virus. Histologic examination of the vaccination site showed a dense infiltration of predominantly helper T-lymphocytes. We conclude that in most of the patients treated active, specific immunization led to a specific antitumor sensitivity.