Plain paraffin was pellets and pellets containing 1% and 5% 20-MCA were implanted in the bladder of 219 C57 X IF and Swiss mice of both sexes. One hundred and fifty four survived and were killed after 40-42 weeks; 56 had been implanted with plain pellet and 98 with pellet containing the carcinogen. Only one bladder tumour (1,8%) developed in the former, whereas 35 tumours (35.7%) were found in the latter group. When a pellet containing 20-MCA was used, the tumour incidence was related to the strain and sex of the animals, rather than to the amount of carcinogen in the pellet. As a matter of fact, irrespective of the concentration of 20-MCA, females C57 X IF gave a significantly higher tumour yiels (87.5%) than males and Swiss mice of both sexes. It is concluded that when bladder implantation is used to induce tumours in mice, the results must be strictly referred to strain and sex. Besides tumours, areas of marked epithelial hyperplasia were seen in bladders implanted with either plain pellet or pellet containing 20-MCA. The possible meaning of this hyperplasia is briefly discussed and the need for further investigation stressed.