Objective To further improve the amount of gadolinium located on tumor, a gadolinium chelate enhanced magnetic resonance imaging pretargeting with avidin-biotin system technique was adopted and the enhancing characteristics of difference of signal intensity at various scan timing were investigated in our experiment. Methods (1) Anti-CEA antibody CL -3 was biotinylated in a mixture with antibody to NHS-LS-biotin with a molar ratio of 1/30-50. (2) After the reaction of GdCl 3 and DTPA-B, the unconjugated gadolinium was removed by chromatography on G-10 column. (3) Steps for pretargeting tumor: First step, McAb-B was injected intravenously into nude mice on the first day. Second step, avidin (Av) and streptavidin (SA) were injected intraperitoneally 24 hours later. Third step, Gd-DTPA-Bt was injected intravenously 48 hours after the first injection. MRI was performed with plain scans, enhanced scans at 20 minutes, 2 hours, 8 hours, and 24 hours after the third step. Signal intensities of tumor and muscles were measured. The pretargeting effect was compared with those of Gd-DTPA-McAb and Gd-DTPA. Results (1) Each monoclonal antibody conjugated with 11-23 biotin and the immunoactivity of biotinylated antibody with 12 biotin/antibody was 94.9%. (2) The enhancing effect of pretargeting approach was tumor specific. Contrarily that of Gd-DTPA was not. (3) The enhancing rate of signal intensity specificity of pretargeting approach was 43%, while that of McAb-Gd-DTPA was 17.9% only, so the enhancing ratio was 2.4. Conclusion Pretargeting approach using avidin -biotin system improves the amounts of gadolinium locating on tumors and yields a specific enhancing effect. It is a promising modality which promotes the ability of Gd labeled magnetic resonance immunoimaging in the detection of colon cancer and its recurrence.