From the reservoirs of the metapleural glands of Myrmicinae ▪ ( Myrmica laevinodis, M. rubida, Atta sexdens, Tetramorium caespitum, Myrmecina graminicola, and Harpagoxenus sublaevis) a strongly acid secretion can be isolated. in Myrmica, Tetramorium, and Harpagoxenus it has no detectable odour, whereas in Atta it smells like honey. The secretion of Myrmica as well as that of Atta is an effective antibiotic against Escherichia coli, Staphylococcus aureus, and Penicillium glaucum. The secretion of both species of Myrmica did not release behavioural reactions in either species when offered on a filter paper or a living ant. Myrmica rubida ▪ on which the secretion of M. laevinodis had been deposited, were as much attacked by M. laevinodis as untreated controls. The metapleural region was not more attractive to Myrmica and Formica ▪ than other regions of the thorax. Formica ▪, with metapleural glands extirpated, were groomed and fed as well as unoperated controls over 10 days by the ▪. Extirpated F. rufa ▪ were recognized and killed by F. polyctena ▪ and vice versa even 6 to 10 days after extirpation. Droplets of the secretion applied to the metathorax of Myrmica were spread over the entire body surface of the same and of other adults, larvae, and over the nesting material. The chemical composition of the metapleural gland secretion of Atta sexdens was investigated. The active antibiotic component was isolated by thin-layer chromatography and gas-liquid chromatography. The identification was made with the help of mass spectrometry and u.v.- and.i.r.-spectroscopy. It was found to be phenyl acetic acid, of which each and stored on the average 1·4 μg. From these results the following conclusion was drawn: the metapleural glands serve as a source of antiseptic; they protect the body surface and the nest against micro-organisms in the species of ants investigated.