Two derivatives of pancreatic ribonuclease and endonuclease of Staphylococcus aureus, insolubilized on corn cob, have been used to reduce the percentage of nucleic acids in single cell protein (SCP) concentrates from yeasts. These derivatives are thermostable and active at 45 degrees C. At these temperatures the contamination by bacteria is negligible. The thermostability is remarkable, since the native nuclease is deactivated at above 39 degrees C. The hydrolysis of the nucleic acids in SCP is carried out first with the ribonuclease derivative followed by the endonuclease derivative. The catalytic activity of the insolubilized derivatives is similar to that of the native enzymes in the hydrolysis of RNA but not of DNA. The percentage of nucleic acids is reduced from 5-15 to 0.5%, with a loss of protein of 6%. These percentages are lower than those previously reported.