Abstract: 1,2,4-Oxadiazoles having as 3- and 5-substituents a hydrogen atom, an aliphatic, cycloaliphatic, araliphatic, aryl or heterocyclic group, or a carbamoyl group of the formula -- CONR.sup.1 R.sup.2 where R.sup.1 & R.sup.2 which can be the same or different, are hydrogen atoms or aliphatic, cycloaliphatic, araliphatic or aryl groups or, taken with the N atom, a heterocvolic ring; provided that at least one of the 3- or 5-substituents is an N-substituted carbamoyl group. Antimicrobial activity, and particularly antiviral, antiparasitic and antibacterial activity is shown in this group. The corresponding oxadiazolins are also described and are useful intermediates in the preparation of the oxadiazoles.