Immunosuppressant n-acyles homoserine lactones

New N-acyl homoserine lactone compounds of the formula (I) in whichR is an acyl group of the formula (II) wherein one of R 1 and R 2 is H and the other is selected from OR 4 , SR 4 and NHR 4 , wherein R 4 is H or 1-6C alkyl, or R 1 and R 2 together with the carbon atom to which they are joined form a keto group, and R 3 containing from 8 to 11 carbon atoms and is optionally substituted by one or more substituent groups selected from halo, 1-6C alkoxy, carboxy, 1-6C alkoxycarbonyl, carbamoyl optionally mono- or disubstituted at the N atom by 1-6C alkyl and NR 5 R 6 wherein each of R 5 and R 6 is selected from H and 1-6C alkyl or R 5 and R 6 together with the N atom form a morpholino or piperazino group, or and enantiomer thereof, with the proviso that R is not a 3-oxododecanoyl group, having immuosuppressant properties are disclosed. The compounds are shown to have an inhibitory effect on lymphocyte proliferation and down-regulate TNF- secretion by monocytes/macrophages in the animal body, including the human body. Pharmaceutical composition comprising N-acyl homoserine lactones are also described. 1