The effect of culture medium from fibroblast cultures of cystic fibrosis (CF) patients and healthy controls on the elemental composition of fibroblasts was investigated by X-ray microanalysis. Exposure of normal fibroblasts to culture medium from CF fibroblasts caused an increase in calcium level. Exposure of CF fibroblasts to culture medium from normal cells caused an increase of the sodium content of CF cells to approximately normal levels; the calcium level of the CF fibroblasts, however, remained abnormally high. The results may indicate that CF fibroblasts lack a factor needed for the regulation of sodium transport. CF fibroblast medium apparently contains a factor that interferes with the regulation of calcium transport.