An in vitro system for evaluating the toxicity of serum from cystic fibrosis (CF) patients is reported. Rat lung epithelial cells were incubated for 18 hours in medium containing varying concentrations of human serum (HS) from CF patients and patients without cystic fibrosis (non-CF). Twenty-four assays were performed for 4 concentrations of each HS. Attachment efficiency (AE), defined as the number of cells attached to the culture surface divided by the number of cells dispensed per culture, was determined for each concentration of HS. Attachment efficiency was always greater when no human serum was present. As the percentage of serum increased, the AE decreased in all cases.Because the AE varied from day to day, the ratio (RAE) of the AE for 0% HS to the AE for 2% HS was calculated for each serum sample. The RAE value was reproducible within 0.07 over a four-week interval for individual serum samples. The higher the RAE value, the more toxic the serum.There is a significant difference (p < 0.05) in the two populations of patients. The serum from the CF patients was more toxic to rat lung epithelial cells than serum from non-CF patients.