The antiproliferative action of the guanine-specific ribonuclease secreted by Bacillus intermedius (binase) was studied in different chicken and mouse cell lines. The proliferation rate of chicken embryo fibroblasts, either normal or Rous sarcoma virus-transformed, was significantly reduced by binase treatment. Among mouse fibroblasts, v-ras-transformed NIH3T3 cells were sensitive to binase, whereas the growth of non-transformed, v-src-transformed or v-fms-transformed NIH3T3 cells was not affected. A 48 h treatment with binase inhibited the Ca2+-dependent K+ current of v-ras-transformed NIH3T3 cells but had no effect on this membrane current in non-transformed and in v-src- or v-fms-transformed NIH3T3 cells. Our results suggest that mammalian cells expressing the ras-oncogene are a potential target for the antiproliferative action of binase.