The human transferrin receptor (hTfR) is responsible for the delivery of iron into the cells through its binding to transferrin. Since cancer cells express much higher level of TfR compared to normal cells, we developed a mouse/human chimeric antibody fusion protein composed of avidin genetically fused to a human IgG3 specific for the hTfR. Our initial goal was to use this molecule (anti-TfR IgG3-Av) as a universal vector to deliver biotinylated agents into cancer cells. However, we unexpectedly discovered that anti-hTfR IgG3-Av alone possesses a strong intrinsic anti-proliferative/pro-apoptotic activity against hematopoietic malignant cell lines. Importantly, this activity can be potentially enhanced by its conjugation to biotinylated therapeutic agents. The goal of the present work is to evaluate the ability of anti-TfR IgG3-Av to deliver biotinylated saporin, a plant toxin that inhibits protein synthesis, into a panel of Chinese Hamster Ovary (CHO) cells expressing no hTfR (CHO-negative), hTfR1 (CHO-hTfR1), or hTR2 (CHO-hTfR2). The use of this panel of cells would allow us to evaluate whether the specificity of our antibody fusion protein is subject to TfR1 alone or whether cross-reactivity to TfR2 is also observed. Using the [3H]-thymidine incorporation assay we found that anti-hTfR IgG3-Av alone has no cytotoxic effects on CHO-hTfR1 cell proliferation as expected. However, anti-hTfR IgG3-Av complexed to biotinylated saporin does show potent cytotoxic effects in CHO-hTfR1. Our results indicate that anti-hTfR IgG3-Av complexed to biotinylated saporin is cytotoxic to cells that only express hTfR1 and not hTfR2. These studies support the use of our fusion complex as an effective delivery agent that can deliver biotinylated agents into malignant cells that express hTfR1.