Transferrin receptor activity as a marker in transitional cell carcinoma of the bladder. An immunohistochemical staining method was used to study transferrin receptors (TFR) in bladder tissue from 10 patients with benign conditions and 33 with bladder cancer. All patients with bladder cancer were followed up by cystoscopy at intervals of 3 months (mean follow-up period 11 months). The relationship between TFR activity and recurrence rates was investigated. It was not possible to demonstrate TFR in normal bladder mucosa except for the proliferating cells of the basal layer. TFR activity in malignant tissue correlated well with the histological grade and pathological stage of the tumour. Patients with low grade superficial tumours showing TFR activity had a higher recurrence rate than those with no TFR activity. It was concluded that TFR activity in low grade superficial bladder tumours is a useful marker for predicting the recurrence rate. 