Protein, DNA, and RNA syntheses were assayed in Madin Darby canine kidney cells (MDCK) treated by bolestaine, a toxic glycoprotein from the mushroom Boletus satanas (single chain, Mr 63,000 +/- 3000, pI 8.3 +/- 0.1, disulphide intrachain bridge) previously shown to be an inhibitor of in vitro protein synthesis. Cellular protein and DNA syntheses are inhibited in a dose-dependent manner after 24 h of incubation, whereas the uptake of the labelled precursors of proteins, DNA and RNA biosyntheses into the cells is not affected. The IC50 of bolesatine for protein synthesis is 0.14 microM in the cell culture medium. RNA synthesis is not inhibited at this concentration. The IC50 for DNA synthesis is 0.32 microM. When galactose is added to the culture medium, it decreases or even abolishes the toxic effects, indicating that it prevents the toxin from binding on the membrane and penetrating inside the cells. The profiles of polysomes in MDCK cells treated with bolesatine, compared to the untreated ones, show an increasing pool of polysomes indicating that the toxin acts on the peptidyl elongation step in the protein synthesis.