The effects of herpes simplex virus type I (HSV-I) on SCE frequencies in human lymphocytes in vitro were investigated. SCE frequencies in controls (mean +/- 1SD 5.2 +/- 1.6) and in cells exposed to the virus for 3 and 6 h (mean +/- 1SD 5.3 +/- 1.6 and 5.8 +/- 1.6 respectively) were about the same. However, cells infected for 9 to 24 h showed a significant increase of SCE frequencies (p < 0.05).