The embryo chicken trachea was used as an in vitro model to study the ciliotoxicity of the volatile part of the particulate phase of cigarette smoke condensate (CSC). We also report on the ciliotoxicity of 11 phenols belonging to the semi-volatile fraction. The particulate phase of CSC was divided into a semi-volatile (SV) and a non-volatile (NV) fraction. The toxicity of CSC, SV and NV was approximately the same, giving ciliostasis within 21-28 minutes at a concentration of 1 mg/ml. The SV-fraction, constituting 12 percent of the particulate phase, was further divided into 4 subfractions: acids, phenols, bases and neutrals. The neutral fraction was significantly (p less than 0.001) more toxic than any of the other fractions, giving ciliostasis after 17 minutes at a concentration of 1 mg/ml. Least toxic was the basic fraction, mainly consisting of nicotine. At a concentration of 3 mg/ml, the mean value for ciliostasis was 64 minutes. From the investigation of phenols it was found that alkylated phenols were more ciliotoxic than phenol. The toxicity was enhanced with increasing chain-length of the substituent, i.e. with increasing lipophilicity.