On the basis of qualitative assays, quiescent lymphocytes have previously been reported to have numerous DNA strand breaks, which are thought to be repaired after mitogenic stimulation by a process associated with poly(ADP-ribosyl)ation. Using alkaline elution, a very sensitive assay for quantifying DNA single-strand breakage, we found no evidence for a high frequency of DNA strand breaks in unstimulated human peripheral blood lymphocytes. No differences in elution profiles were observed between unstimulated lymphocytes and lymphocytes 4 or 48 h after addition of the mitogen phytohemagglutinin (PHA). Furthermore, addition of 3-aminobenzamide (3AB), an inhibitor of poly(ADP-ribose) synthetase, or aphidicolin, an inhibitor of DNA polymerase alpha, did not increase the amount of DNA eluting from the filter after PHA stimulation. In contrast to reported studies of mouse splenic lymphocytes, we found that human lymphocytes were able to replicate and divide in the presence of the ADP-ribosylation inhibitor. Human lymphocytes were also capable of proliferating in nicotinamide-free medium, with or without 3AB, indicating that ADP-ribosylation is not a requirement for lymphocyte differentiation. We therefore consider it unlikely that peripheral human lymphocytes contain significant numbers of strand breaks that play any role in their stimulation or differentiation in response to PHA.