Internalization of 27.B1 and 27.F7 antibodies into breast cancer cells. 27.B1(B) and 27.F7 (C) antibodies were internalized into MCF7 cells. Antibody-treated cells were evaluated for intracellular staining with the aid of laser scanning confocal microscopy. Warming of the antibody-bound cells to 37°C for 30 minutes revealed intracellular staining of the MCF7 breast cancer cell line. In contrast, no intracellular staining was visualized with the non-internalizing control antibody (human myeloma IgM).