Unscheduled DNA synthesis in human peripheral blood lymphocytes has been compared following exposure of cells at various dose levels to UV light, γ-rays, methyl methanesulphonate (MMS) and nitrogen mustard (HN 2). Lymphocytes in G 1 were exposed to these mutagens and then incubated in a medium containing [ 3H]thymidine. For each mutagen the dose response and the temporal pattern of synthesis has been determined by scintillation counting of TCA (trichloroacetic acid)-extracted samples. In each case synthesis was found to initially increase with increasing dose, and following both types of radiation the initial increase was exponential. The system was found to be very sensitive to UV irradiation and unscheduled DNA synthesis has been detected at doses of 2 erg/mm 2, reaching a maximum at 40 erg/mm 2. The value obtained at dose levels above 40 erg/mm 2 was found to be constant for doses up to 5600 erg/mm 2 and then declined. Similar reductions in the amount of synthesis at high dose levels were also found with the chemical mutagens. For estimations of the duration of the reaction, cells were harvested at various times up to 22 h post-treatment. A parallel response was found for UV- and HN 2-treated cells; for all doses considered, the reaction was largely completed within 6 h following exposure. This temporal pattern contrasted with the response following γ-irradiation and MMS treatment where synthesis continued for the whole of the incubation time considered. With γ-rays, the first 60 min following exposure was characterised by a phase of rapid synthesis, the amount of label incorporated increasing linearly over this period. The rate of incorporation between 1 and 22 h, however, although constant, was reduced to 25% of the early maximum rate. This biphasic response confirms and extends earlier studies and it is suggested that the early incorporation may be associated with the rejoining of radiation-induced single-strand breaks in the DNA. In the case of MMS a somewhat similar temporal pattern to that found with γ-rays was observed. Consideration of the initial rates of the reactions following treatment swith each of the mutagens, indicates that the unscheduled synthesis of DNA is an enzyme-limited reaction, possibly involving a different enzyme in the case of each mutagen.