Cytogenetic effects of two clastogenic antitumor agents, actinomycin D and bleomycin sulfate were assessed in G2 phase of cell cycle in two heterohybridoma cells, and two non-hybrid cell lines. Since heterohybridoma cells carry chromosome constitutions from different species, their responses to various environmental stimuli may vary and depend on their chromosomal origins. Two hybridoma cell lines, HF2x653 and F3B6, and two nonhybridoma cell lines, WIL2 and NS-1, as their parents were treated with bleomycin sulfate and actinomycin D in G2 phase of the cell cycle. Metaphase spreads were prepared and various types of chromatid aberrations were scored. The frequencies of chromatid aberrations of hybrid lines in both treatments were between the frequencies of WIL2 and NS-1. HF2 showed same chromosome aberration frequency with WIL2 and F3B6 with NS-1. The results indicated that chemosensitivity of hybridoma cell lines investigated in this study was nearly similar to their parental cell lines.