Esters or amides of a peptide, preferebly a dipeptide, consisting essentially of natural or synthetic L-amino acids with hydrophobic side chains were found to have specific cellular toxicities. Preferable amino acids of the peptide are leucine, phenylalanine valine, isoleucine, alanine, proline, glycine or aspartic acid beta methyl ester. Preferable dipeptides are L leucyl L-leucine, L-leucyl L-phenylalanine, L-valyl L-phenylalanine, L-Leucyl L-isoleucine, L-henylalanyl L-phenylalanine, L-valyl L-leucine, L-leucyl L-alanine, L-valyl L-valine, L-phenylalanyl L leucine, L prolyl L-leucine, L-leucyl L-valine, L-phenylalanyl L-valine, L glycyl L-leucine, L-leucyl L-glycine, glycyl L-phenylalanine and L-aspartyl beta methyl ester L-phenylalanine. Most preferable dipeptides are L-leucyl L-leucine, L-leucyl L-phenylalanine, L-valyl L-phenylalanine, L-phenylalanyl L-leucine, L-leucyl L-isoleucine L-phenylalanyl L-phenylalanine and L-valyl L-leucine.The ester or amide of the dipeptide is most preferably alkyl, aralkyl or aryl a preferred alkylester is a methyl ester and may also be an ethyl ester or alkyl of up to about four carbon atoms such as propyl, isopropyl, butyl or isobutyl. Yet larger alkyl substituents may also be functional judging from the beta naphthyl substituent which is functional in certain embodiments.These alkyl, aryl or arylkyl esters and amides of dipeptides consist essentially of amino acids with hydrophobic side chains may be used to deplete cytotoxic T-lymphocytes or natural killer cells from organisms, cell populations or tissues.