This article presents research on the effect of microbial transglutaminase on the properties of Turkish drinking yoghurt Ayran. Transglutaminase (TGase) enzyme was added to Ayran milk at different production steps, after pasteurisation and together with starter culture addition, and two different incubation times (10 min and 1 h) were used. Three TGase-treated Ayran samples and control sample were analysed on 1st, 10th and 20th days of storage. TGase addition did not cause significant changes on chemical properties of Ayran (P > 0.05). However, physical properties of Ayran were improved by TGase throughout the storage period. Compared to the untreated control sample, pretreatment of milk with enzyme increased the viscosity dramatically (P < 0.05) and prevented serum separation. Scanning electron microscopic studies revealed that transglutaminase treatment of milk led to more regular distribution of proteins in Ayran gel and decreased gel permeability. TGase treatment did not show any unfavourable effect on the sensory properties of the final product.