A method of examining cytological material during fibreoptic bronchoscopy using a methylene blue (MB) stain was assessed in 164 consecutive fibreoptic bronchoscopies where cytology specimens were taken. The MB method provided an immediate positive diagnosis in 86% of bronchoscopically visible tumours. Subsequent histology provided a positive diagnosis in 69%, conventional brush cytology in 81% and trap cytology in 77%. The MB method produced no false positive diagnosis of malignancy and the tumour cell type identified by MB stain agreed with the histological cell type in 72% of cases. This technique is considered to be sufficiently specific to provide a method of controlling the quality of specimens taken at bronchoscopy, for further analysis in the laboratory.