Yeast grown on hydrocarbons according to the n-paraffin process of The British Petroleum Company Ltd. was fed to groups of male and female rats at dietary levels of 10, 20 and 30% for 1 yr. Two control diets were used, one being similar to the yeast diet but with soya-bean meal substituted for yeast and the other being the institute's stock diet. The inclusion of the yeast in the diet did not affect mortality, general condition or behaviour, gain in body weight, or food intake. Haematological values and blood biochemistry were not affected by the consumption of the yeast. There was no evidence of impaired kidney function or of changes in urine composition. No effects related to treatment were revealed by the weights of the principal organs nor by extensive gross and microscopie pathological examinations. This investigation indicates, therefore, that yeast grown on pure n-paraffins by the BP process does not induce any deleterious effects when fed to rats at dietary levels up to 30% for 1 yr.