Cigarette smoke is carcinogenic in animals, and most of this carcinogenic property is recovered in non-polar subfractions of the neutral fraction. Although cigarette smoke condensate is mutagenic following activation by metabolic enzymes found in microsomes or the S9 fraction, it has not been reported to act as a direct mutagen in systems used thus far. In addition, the fractions determined to be most carcinogenic have not been found to be the most mutagenic. We have employed Chinese hamster ovary (CHO) cells containing a human chromosome 11 (termed AL hybrid cells) as a more sensitive detection system for mutagenesis in an effort to better define the direct and indirect mutagenic properties of cigarette smoke condensate and its fractions. The inhibitory dose (ID)50 value for N-methyl-N-nitro-N-nitrosoguanidine (MNNG) (0.2 ug/ml) in medium containing 8% fetal calf serum (FCS) the known direct mutagen was similar to previous reported values. The ID50 for MNNG in medium lacking FCS was 0.6 ug/ml in two separate experiments (N=8). At this concentration, there was a 300% increase (p<0.0005) in survivors following incubation with the al antibody and complement. The ID50 for cigarette smoke condensate (CSC) in the absence of serum is about 45 ug/ml. CSC is not significantly mutagenic at the ID50 concentration, but causes a 60% increase (p<0.02) in survivors (Presumably mutated) at 60 ug/ml and a 290% increase (p<0.0005) at 90 ug/ml. We have conducted dose response experiments with CSC fractions and have determined ID50s for the acid, neutral and basic fractions of 5, 60 and 250 ug/ml, respectively. The presence of 8% fetal calf serum in the medium reduces the ID50 of CSC and of each fraction to 130, 30, 50 and 360 ug/ml, respectively.