Abstract: Single-cell proteins are the dried cells of microorganism, which are used as protein supplement in human foods or animal feeds. Microorganisms like algae, fungi, yeast and bacteria, utilize inexpensive feedstock and wastes as sources of carbon and energy for growth to produce biomass, protein concentrate or amino acids. Since protein accounts for the quantitatively important part of the microbial cells, these microorganisms, also called single cell protein as natural protein concentrate. With increase in population and worldwide protein shortage the use of microbial biomass as food and feed is more highlighted. Although single cell protein has high nutritive value due to higher protein, vitamin, essential amino acids and lipid content, there is a doubt to be replaced to the conventional protein sources due to their high nucleic acid content and slower in digestibility. They also may be considered as foreign material by body, which may subsequently results in allergic reactions.