A single cell protein preparation derived from cells grown on a chemically pure hydrocarbon fraction was given daily to 20 healthy adults during 6 weeks. Clinical observations showed little evidence that the daily intake of 20 g of the product (comprising 12-15% of the total nitrogen intake) caused any severe side effects. Dietary studies showed that the main problem of the product from the dietetic point of view was the taste, which needed further development. Biochemical investigations revealed a significant but transient increase of the serum uric acid level during the first 2 weeks. With respect to the serum transaminases (SGOT and SGPT) there was a moderate increase which, however, never surpassed the upper limit of the normal values.