Single cell proteins prepared from yeast or bacterial cells grown on a chemically pure hydrocarbon fraction were fed to male and female rats either as the sole sources of protein at a dietary protein level of 20 % or as supplements to a commercial stock diet at two levels providing 15 and 25 %, respectively, of single cell protein. Two control diets were used. One was a synthetic casein containing diet and the other a commercial stock diet. With the exception of an increased occurrence of renal calcification yeast diets did not affect mortality, general condition or behaviour of the animals. Yeast as the sole source of protein produced a lower growth rate than the casein and stock diets. This could be due to a deficit in sulfur-containing amino acids. Adverse effects on the health of rats fed bacterial single cell protein as the sole source of protein were observed. These were arrested growth, weight losses, bleedings at nose and eyes and increased mortality. At the histological examination of organs from these rats, severe pyelonephritis, nephrocalcinosis and testicular atrophies were seen. Blood from these rats showed a changed neutrophil: lymphocyte ratio.