The immunoconjugate XMMCO-791/RTA consists of ricin A chain bound to a murine monoclonal antibody MoAb 791T. This monoclonal antibody (MoAb) binds to a glycoprotein of 72 kD, which is expressed on human colorectal carcinoma, ovarian carcinoma, and osteogenic sarcoma. XMMCO-791/RTA was tested in a Phase I trial with proposed dose escalation steps of 0.02, 0.04, 0.15, and 0.2 mg/kg per day. Twelve patients with metastatic colorectal carcinoma were treated at 0.02, 0.03, and 0.04 mg/kg per day dose levels administered over 1 hour on days 1-5. Study-related toxicities were hypotension (6 patients); greater than 10% weight gain (6 patients); peripheral edema (9 patients); fever (4 patients); confusion (3 patients); diarrhea (3 patients); proteinuria, as identified by dipstick (3 patients), greater than 0.6 mg/dl decrease in serum albumin (11 patients); greater than 25% decrease in oncotic pressure (10 patients), and a decrease in ionized calcium (8 patients). Six patients received a second course of treatment. HAMA levels developed in 9 patients and titers increased with number of courses administered. Decreased overall toxicity, in comparison to the first course, was noted, but one patient had an allergic-type response (hypotension, crushing chest pain, diaphoresis) after the test dose of the second course (HAMA level > 10,000 IgG). Life-threatening toxicity in the form of fluid shift, resulting in noncardiac pulmonary edema and third-spacing occurred after course 1 in 1 of 3 patients at the 0.04 mg/kg per day level. No further dose escalation was attempted and no antitumor activity was seen.