The filter elution technique was used to assay Co-60 g ray-induced DNA single-strand breaks (SSB) in EL 4 mouse leukemia cell and spleen lymphocyte. The lymphocytes were stimulated with lipopolysaccharide (LPS, 20 mug/ml) to label [3H] thymidine. EL 4 cells and lymphocytes in suspension were exposed at 0degree C to gy, 1 gy, 5 gy, 10 gy of Co-60 radiation and elution procedure was performed at PH 12.1. The number of DNA single-strand breaks increased with increasing doses of g rays. The strand scission factor (SSF) was estimated in each experiment (eluted volume 21 ml. The slope for EL 4 cells was 0.01301+/-0.00096 gy-1(n=5) and the slope for lymphocytes was 0.01097+/-0.00091 gy-1(n=5). The slopes were significantly different (p<0.005). Thus EL 4 cells were more sensitive to induction of DNA SSB by ionizing radiation than lymphocytes.