Incubation of [125I]26b with a panel of human breast tumor cells and subsequent cell lysis and electrophoretic protein separation by SDS-PAGE showed that these cells contain up to four different molecular species of cathepsin L and two different forms of cathepsin B. The knowledge of the radiotracer's specific activity enabled the calculation of quantitative amounts of the different cathepsin B and L species in these cells in pmol/mg of cellular protein. This evidence concerns the gene CTSL and breast neoplasm.