The effects of the antitumor antibiotic, quinocarmycin citrate (KW 2152), on L1210 cells were studied in vitro. The cellular growth was completely inhibited at 10(-6) M KW 2152, and after 2 days no viable cell was seen. The incorporation of 3H-thymidine, 3H-uridine, or 3H-leucine into the acid-insoluble fraction was not affected at 10(-4) M for 1 h; however, when the cells were treated with 10(-6) M for 24 h, the radioactivity appearing in the acid-insoluble fraction was reduced to 20%, 30%, and 48%, respectively, of the control. The single strand scission of the DNA of L1210 cells was seen at 10(-7) M for 24 h, as revealed by an alkaline, sucrose density gradient. However, no damage to plasmid pBR322 was observed even at 10(-6) M KW 2152 for 24 h, as revealed by 0.8% agarose gel electrophoresis, indicating that some soluble factors of the cells might contribute to the damage to the DNA of L1210 cells. The processing of pre-rRNA of the cells was not inhibited at 10(-6) M of the drug for 24 h of incubation.