The human colorectal carcinoma cell line SW1116 under optimal growth conditions synthesized and shed antigens bearing the monoclonal antibody-defined carbohydrate determinant CA 19-9. Antigen expressing CA 19-9 in cell culture supernatant was quantitated by an immunoradiometric assay for CA 19-9. Injection of SW1116 cells s.c. into athymic BALB/c mice resulted in the growth of moderately differentiated tumors possessing a distinct morphological resemblance to a typical adenocarcinoma of the colon. Intervals to tumor appearance were dependent on inoculum dose, but 95% of mice at both 5 × 106 and 107 cells/mouse developed tumors within 14 to 21 days. CA 19-9 antigen was detected in the sera of all nude mice with SW1116 tumors, and antigen concentration correlated (r = 0.77) with tumor volume throughout the 9-week study. The half-life of this antigen in serum following tumor excision from nude mice was 6.5 ± 1.5 (S.D.) hr. Carcinoembryonic antigen was also detected in serum from mice bearing SW1116 tumors by an immunoradiometric assay for carcinoembryonic antigen, but its concentration correlated (r = 0.86) with tumor volume for only the first 4 weeks of tumor growth. Significant levels of endogenous immunoglobulin G1 and immunoglobulin G3 antibodies to CA 19-9 antigen were found in the serum of nude mice with SW1116 tumors by radioimmunodiffusion, but no apparent relationship between antibody titer and tumor growth or CA 19-9 antigen level in serum was evident. This tumor model may be useful in devising radioimmuno detection and immunotherapeutic strategies for primary and metastatic human colon carcinomas.