The cell-type specificity of the toxin ricin, which ordinarily binds, enters, and kills cells through receptors containing galactose, has been altered by covalently binding it to a monoclonal antibody and by reversibly binding it to lactose. The antibody, a monoclonal rat IgG2b directed against the Thy 1.2 antigen, provides ricin with a new binding site for the murine thymus cell surface. Addition of lactose saturates the galactose-binding site on ricin and inhibits ricin from binding and killing cells via the galactose-containing receptors. The antibody-ricin hybrid protein, anti-Thy 1.2-ricin, formed with a thioether linkage, has been purified by size exclusion and affinity chromatography. When assayed by inhibition of protein synthesis of EL-4 cells, which express the Thy 1.2 antigen anti-Thy 1.2-ricin is equally as toxic as ricin on a molar basis. The hybrid protein toxicity is unchanged in the presence of 100 mM lactose, whereas unmodified ricin toxicity is reduced to 1% of its toxicity in the absence of lactose. This demonstrates the altered receptor specificity of the ricin hybrid. The cell-type specificity of the anti-Thy 1.2-ricin inhibition of protein synthesis correlates with the presence of the Thy 1.2 antigen. Anti-Thy 1.2-ricin at 4 microgram/ml in the presence of lactose inhibits protein synthesis within 3.5 hr by 60-80% in EL-4 cells but does not affect Thy 1.1 alloantigen and HeLa cells that lack the Thy 1 antigen. Anti-Thy 1.2-ricin in the presence of lactose selectively kills EL-4 cells at concentrations that do not kill AKR-K36 cells. This selectivity, expressed as the ratio of anti-Thy 1.2-ricin concentrations required to kill 40% of both cell types, is 700. Ricin-monoclonal antibody hybrids of this type combine a high degree of cell-type selectivity and toxicity and may have pharmacologic utility as antitumor reagents.