The invention provides a method of treating T-cell mediated diseases and a method of inhibiting the activation of T-cells using certain diketopiperazines. The invention also provides methods of synthesizing diketopiperazines and pharmaceutical compositions comprising certain diketopiperazines. The invention further provides methods of making improved pharmaceutical compositions of proteins and peptides by either increasing or decreasing the content of diketopiperazines in the compositions and the resultant improved pharmaceutical compositions.
