131I-antibodies (Ab) to human organ-specific intestinal antigen--beta 1-MA (maconial antigen) were injected intravenously to nude mice to whom various human tumors including colon tumors had been transplanted. Ab distribution in recipient tissues and in tumors was determined over time with the help of a gamma-counter. Ab to beta 1-MA was shown to localize specifically in colon tumors. 131I-Ab accumulation in this tumor exceeded the background level in tumor-bearing tissues. The excess increased with time as a result of slower Ab clearance of colon tumors as compared to normal tissues. beta 1-MA was detected immunohistochemically in the mouse colon epithelium however elevated accumulation of Ab to beta 1-MA injected into the blood flow in the intestine was undetectable.