The ImmunoCyt assay (Diagnocure Inc., Québec, Canada) is a new immunocytological fluorescence test for identifying two different mucins and a high-molecular-weight glycosylated carcinoembryonic antigen (CEA) present in tumours originating from transitional epithelial cells. The test promises a higher diagnostic sensitivity in transitional cell carcinoma (TCC) of the bladder than voided urine cytology. Our study was designed to evaluate this test especially for TaG1 carcinomas, which are characterised by a low detection rate in urinary cytology. A total of 121 spontaneous urine samples of 92 patients (age range 28 to 86, mean 62.5 years) were examined. The samples were taken from patients suspected of having TCC (41 out of 121) or tumor recurrence (46 out of 121), or who were part of a follow-up protocol (34 out of 121). Cystoscopy was practiced in all patients. The ImmunoCyt test was carried out according to the manufacturer's protocol. For cytology cytospins were made from the same urine samples and stained according to the method of Papanicolaou. One hundred and thirteen specimens could be evaluated. In 87 cystoscopy and/or histology were negative. There was histological evidence of 7 pTaG1, 4 pTaG2, 8 pT1G2/G3 and 7 pT2G2/G3 TCC. As for ImmunoCyt and cytology, specificity was 83.9% and 91.9%, respectively. A combination of either test indicated 81.6% specificity. The sensitivity amounted to 38.5% and 34.6%, respectively, and the combined sensitivity to 53.8%. The sensitivity for TaG1 carcinomas was 14.3% each, for TaG2 carcinomas 25% and 50%, for T1G2/G3 carcinomas it amounted to 37.5% each, while for T2G2/G3 carcinomas it was 71.4% and 42.9%, respectively. The higher sensitivity of the ImmunoCyt test as compared to urinary cytology renders improved identification of exfoliated tumour cells in bladder cancer possible. In our study, however, the expected increase in detecting TaG1 carcinomas was not found. Because of its lower specificity, the test should only be used in combination with voided urine cytology. On account of its low sensitivity, the ImmunoCyt test cannot replace cystoscopy (with biopsy) in the diagnosis and monitoring of bladder cancer.