Fairley and Kilgour, in 1966, grouped twenty amino acids into eight categories.

Simple amino acids: No functional group in their side chain.

Example: Gly, Ala, Val, Leu, and Ileu

Hydroxy amino acids: Contain hydroxy group.

Example: Ser and Thre

Sulphur amino acids: Contain sulphur group.

Example: Cys and Met

Acidic amino acids: Contain carboxyl group.

Example: Asp and Glu

Amino acid amides: Derivatives of acidic amino acid whose one COO⁻ group transformed into an amide group .

Example: Asn and Gln

Basic amino acids: Contain amino group.

Example: Lys and Arg

Heterocyclic amino acids: Have a side chain ring, which possesses one atom other than carbon.

Example: Trp, His, and Pro

Aromatic amino acids: Have benzene ring

Example: Phe and Tyr

Delimitations of this classification are as follows:

1. Trp: This can also be included under aromatic.
2. His: This can also be included under basic amino acids.