A microspectrofluorometer has been used for kinetic studies of the decrease of benzo(a)pyrene and benzo(k)fluoranthene fluorescence. This decrease is observed for single living cells: L cells and human peripheral blood monocytes, after their incubation which culture medium containing these compounds and washing the petri dish with fresh medium. The entire fluorescence spectra is recorded at given time intervals in order to watch at some eventual spectral modification. The fluorescence decrease is monoexponential and its parameters are computed with a program based on the least squares fit method. Such determination shows no difference between the calculated rate constants of metabolisation for B(a)P and B(k)F and, as long as we consider L cells with a similar morphological shape, only statistical fluctuations of the rate constants of metabolism are observed. As compared, monocytes show faster kinetics of the decrease of the B(a)P intracellular fluorescence due to B(a)P metabolism, and also a more reached dispersion of the values of the rate constant than the one observed for L cells indicating some heterogeneity in the monocyte population of each donor.