Abstract The effect of 20, 40 and 80 μg per 100 ml concentrations of lead on the in vitro senescence of fetal human diploid fibroblasts IMR-90 was determined. The areas and dry mass of the cell, nucleus and nucleolus were measured at early, middle and late passages. There was a decrease in total population doublings as the concentration of lead in the medium was increased. Although there was a decrease in the number of nucleoli per cell with successive doublings, there was no difference between controls and lead-treated cells. There was an increase in nucleolar dry mass as the cells aged and this was most noticeable in the 40- and 80-μg groups. There were no noteworthy changes in nuclear and cellular areas and dry mass with respect to lead treatment. The results are discussed and it is concluded that even subclinical concentrations of lead cause an acceleration of cellular aging in vitro.