The toxic concentration of different heavy metal salts was determined in normal stimulated human lymphocyte cultures and was found to be 3 X 10(-3), 1 X 10(-2) and 5 X 10(-4) for zinc chloride, lead acetate and cadmium chloride respectively. Furthermore 3 subtoxic doses of each salt (2, 10 and 100 times less than the toxic dose) were added to 48- and 72-h cultures at 0 h and 24 h after initiation. Chromosome preparations were made and 100 well spread metaphases from each culture were analysed for the presence of numerical and structural aberrations. The most common aberration found for all tested metal salts was the occurrence of chromosome fragments. Dicentric chromosomes were only recorded in lymphocyte cultures treated with the lowest concentration of zinc chloride (3 X 10(-5) M) added at time 0, regardless whether the cultures were fixed after 48 or 72 h.