The streptavidin-biotin system has been used to immunotarget whole ricin to tumor cells in a system that overcomes ricin-nonspecific cytotoxicity. Biotin was linked to ricin via a disulfide-containing reagent, sulfosuccinimidyl-2-(biotinamido)ethyl-1,3'-dithiopropionate. The product, biotinyl-S,S-ricin (b-ricin), retained most of its in vitro cytotoxic activity against human epidermoid carcinoma (KB) cells. Complexing b-ricin to streptavidin resulted in greater than 99% loss of its cellular toxicity which is associated with loss of cell-binding activity. The streptavidin-b-ricin complex could, however, be targeted to KB cells via the biotinylated monoclonal antibody 108 which is specific to the epidermal growth factor receptor overexpressed on KB cells. The complex did not regain its activity if the specific antibody was not biotinylated or if the biotinylated antibody was of a different specificity. Streptavidin is thus used to block b-ricin, presumably due to a steric restraint of the streptavidin on the ricin B-chain, and to bridge it to biotinyl antibody recognizing the target cell. Avidin could not replace streptavidin in this system since a complex between b-ricin and avidin retained a major part (60%) of ricin cytotoxic activity. This is attributed to the nonspecific binding of avidin to cells in vitro, including the KB cells. It is suggested that b-ricin is blocked by both streptavidin and avidin, but once the complex gains access to the cell surface, its cytotoxic activity is specifically retrieved.