Previously we reported haematopoietic death as an effect of tritiated water (HTO) in drinking water in the concentration range from 5.92 x 10(11) to 1.85 x 10(10) Bq/dm3. In the present study the effects of HTO in a lower concentration range from 9.25 x 10(9) Bq/dm3 (0.240 Gy/day) to 3.70 x 10(8) Bq/dm3 (0.096 Gy/day) are reported. Female (C57BL/6N and C3H/He)F1 mice were maintained on drinking water containing various levels of HTO. Mice survived for > 150 days with a high incidence of tumour development (70 to 80%). In the dose-rate range from 9.25 x 10(9) Bq/dm3 (0.240 Gy/day) to 1.85 x 10(9) Bq/dm3 (0.048 Gy/day) the main cause of death was thymic lymphoma. However, at a dose-rate of 9.25 x 10(8) Bq/dm3 (0.024 Gy/day) the incidence of thymic lymphoma sharply decreased, while the incidence of other tumours increased. The tumour type became more diverse at lower concentrations of HTO. The latent period of tumour development was shorter and the life-shortening effect was more marked by 3H beta-irradiation in this study than b X- or gamma-irradiation reported in other investigations.