The authors tried to determine a dose-effect relation between the concentration of Bleomycin and the number of chromosomal aberrations "in vitro" in human lymphocytes during 48 h cultures. Concentrations of 1 gamma/ml, 2 gamma/ml, 10 gamma/ml and 20 gamma/ml were used. The number of normal cells seems to decrease according an exponential function; at a concentration of 20 gamma/ml, 15 p. 100 of the cells were apparently normal and this rises the problem of chemotherapeutic agent resistance. The number of one hit and two hit damages increases according to the dose of Bleomycine but it was not possible to determine a mathematic relation of the phenomenon. Like viruses and others chemicals, Bleomycin was responsible for "pulverisation" of chromosomes in several cells.