Sprague Dawley rats were fed with yeast (Candida Maltosa) obtained by fermentation of n-Paraffins F.U. grade (C12-C19). The yeast was incorporated in the diet at 7.2, 18.4 and 34.5% by weight. Each diet was isocaloric and isoproteic with the others and with the standard diet. The yeast supplied 20, 50, 80% of the proteins of the diet respectively. 65 rats per sex per group were selected at random from over 1000 rats and assigned to each of the 4 diets for the carcinogenicity study; 57 rats per sex group were selected at random from the same 1000 rats and assigned to each of the 4 diets for the long term study. In long term study the rats were sacrificed at 3, 6, 15 and 24 months. In the carcinogenicity study the animals were kept till less than 10% of the starting number was surviving; the experiment lasted 30 months. Animals dead spontaneously or killed at the end of the trial were autopsied and the main organs fixed for histological examination. Lesions and tumours were classified. Biochemical, haematological and autopsy variations at the times of sacrifice were observed in the long term study. The experiment showed no pathological differences between controls and animals treated with 20 and 50% proteins from yeast. The group fed with 80% single cell protein showed a significant increase of malignant lymphomas incidence.