In the context of the search for new sources of protein to meet the growing demand for alternatives to animal sources, single cell proteins (SCP) represent a fast-growing sector. However, due to their high content in nucleic acids, their uses in human food are limited. SCP from yeast (Y1) and bacteria (B1) with decreased nucleic acid content have been produced and our objectives were: 1) to evaluate their protein quality and 2) to determine the effect of 50 days of SCP intake on health-related parameters in rats. In a first study, protein efficiency ratio (PER) was determined in young Wistar rats fed a diet containing either casein (control), Y1 or B1. Faecal digestibility was evaluated thanks to 15N-labelled proteins and PDCAAS of Y1 and B1 were calculated. In a second study, Wistar rats were fed a casein, Y1 or B1 diet during 50 days. Food intake and body weight were daily measured and urine and plasma were collected at 0, 25 and 50 days. PER of B1 (2.2±0.1) and Y1 (1.9±0.1) were lower than casein (2.5±0.1, P<0.001). Faecal digestibility of Y1 (76.6±5.1%) and B1 (79.7±4.8%) were similar but low compared to casein (93.7±1.1%, P<0.001). However, thanks to a balanced indispensable amino acids profile, PDCAAS was good (0.9, leu) for Y1 and excellent (1.1) for B1. In addition, 50 days of Y1 or B1 diet did not alter food intake, body weight or body composition, and plasma metabolic parameters and inflammatory markers were similar to those of casein-fed rats. Increased urinary uric acid was observed with Y1 diet but without hyperuricemia, suggesting an efficient excretion of uric acid. In conclusion, despite a moderate PER, protein quality of Y1 and B1 was very good and no deleterious effect has been observed after mid-term consumption of these SCP.