We have developed a sensitive ELISA using MAb 4D3 for the detection of a novel epitope on Small Intestinal Mucin Antigen (SIMA) and report here that SlMA is present in the serum of patients with colorectal cancer. SlMA has been shown to occur in tissue from a high proportion of patients with colorectal cancer. SlMA derived from serum was similar to tissue-derived SIMA both eluted in the void volume of a Superose 6 column indicating a molecular weight above 5,000 kDa and they exhibited similar buoyant densities on CsCl gradients. The ELISA was most reliable after pre-treatment of serum with 0.4 M perchloric acid to remove interfering substances. The upper limit for SlMA in normal serum was set as the mean plus 2 standard deviations determined from a group of 97 healthy control subjects. In a sample of I13 patients with colorectal cancer, SlMA serum levels were elevated in 15% of patients with Dukes' Stage A, 38% with Stage B, 32% with Stage C and 75% with Stage D colorectal cancer. SlMA serum levels were compared with those of the widely used tumor marker, carcinoembryonic antigen (CEA). The SlMA assay detected a significant number of sera that were not detected by the test for CEA. We propose that SlMA will prove to be a valuable serological tumor marker, in combination with CEA and other tumor markers, for the detection of colorectal cancer.