MOv18, a non-internalizing monoclonal antibody (MAb) with restricted tumor specificity, was conjugated to ricin toxin (RT). According to their ability to bind to galactose residues of Sepharose 6B, the immunoconjugates were fractionated into Bound and Unbound MOv18-RT. The two conjugates could be distinguished by SDS-PAGE, in vivo toxicity and agglutination capability. When the binding activity of both fractions was compared by solid-phase RIA to that of native MAb, it proved to be similar on the relevant target cells but significantly increased on the non relevant cells. On the latter, galactose totally cancelled the binding of the Unbound immunoconjugate, whereas it could only partially reverse that of the Bound MOv18-RT. By in vitro cytotoxic activity, either in the presence or absence of galactose, only a slight selectivity for relevant versus non-relevant target cells was observed for both conjugates. It seems that in the presence of a MAb which is incapable of internalization, the conjugate cytotoxicity could only be attributed to RT, with a loss of the MAb's specificity.