The toxic A chain of ricin and the Fab' fragment of antibody directed against human immunoglobulin (Ig) have been disulfide linked via their intrinsic sulfhydryl groups. This Fab'-A chain conjugate retained the activities of its component parts. It produced effective inhibition of protein synthesis in a cellfree rabbit reticulocyte lysate system and bound tightly ti Ig determinants on human cells. Attachment of the intact molecule to the cell surface was revealed by using fluorescein-labeled antibodies directed against both its Fab' and A chain halves. The Fab'-A chain conjugate was evaluated for cytotoxicity by using human, surface Ig positive target cells and surface Ig negative control cells. In vitro toxicity was entirely selective since the conjugate produced inhibition of de novo protein synthesis, impedence of growth, as well as death and lysis, only for cells possessing surface Ig determinants. Moreover, the effects on Ig positive cells were abrogated by the addition of free human IgG that competitively blocked the Fab' combining site of this conjugate and prevented cell surface binding. Addition of lactose, which inhibits binding of whole ricin, did not influence the action of the Fab'-A chain conjugate. These results verified that the specific binding site on the antibody half of the molecule could function as a directional carrier that facilitates A chain entry into the cytoplasm. Expression of cytotoxic effects was thereby restricted exclusively to cells bearing the appropriate target site. This new conjugate molecule thus possessed both the absolute specificity of the antibody and the potent lethality of its parent toxin.