With the advent of hybridoma technology, monoclonal antibodies (MAbs) can be produced against specific human tumor cell-surface antigens. 44X14, an MAb produced in our laboratory by the immunization of BALB/c mice with colon carcinoma cells, exhibits high affinity for breast, lung, and colon carcinomas. However, this MAb shows little evidence of in vitro cytotoxicity. To enhance the potency of this MAb, it was coupled to the A chain (RTA) of the castor-bean protein ricin. Ricin is composed of two subunits--RTA, which binds to ribosomes and inhibits protein synthesis, and the B chain (RTB), which binds to galactose residues on all human cells and facilitates entry of RTA into the cell. By chemically separating RTA from RTB, RTA can then be coupled to MAb 44X14 so as to redefine its specific toxicity. This immunotoxin 44X14-RTA was assayed for protein-synthesis inhibition in HT-29 colon carcinoma and HT-1080 sarcoma cells by [3H]leucine uptake. Intact ricin (RTA + RTB) inhibited protein synthesis by 50% at concentrations of 0.76 and 4.8 ng/ml in HT-29 and HT-1080 cells, respectively. MAb 44X14 showed the same level of inhibition on HT-29 cells at 6.4 micrograms/ml, whereas the immunotoxin MAb 44X14-RTA showed 50% inhibition at 0.15 micrograms/ml. No effect of either MAb 44X14 or MAb 44X14-RTA at concentrations up to 200 micrograms/ml was seen on HT-1080 cells. Thus, the coupling of RTA to carcinoma-specific MAb 44X14 increased its potency 50-fold without increasing its nonspecific binding to cells that do not contain the appropriate antigen.(ABSTRACT TRUNCATED AT 250 WORDS)