Casein solutions (5% w/v) were treated with microbial transglutaminase (MTG) and glucono-delta-lactone (GDL) under varying conditions in order to obtain gels. Storage modulus (G') and gelation time of the gels were measured by oscillation rheometry, while protein cross-linking was determined by gel permeation chromatography. The addition of only GDL to milk resulted in very weak gels, while MTG on its own was not able to create gel networks. Simultaneous action of both ingredients led to gels, the firmness of which was linearly related to the added amount of MTG, but passed through a maximum with rising GDL concentrations. Using chromatographical analysis, increasing G' values were interrelated with the formation of MTG-induced oligomers. The gelation time was directly proportional to the GDL concentration but not influenced by the addition of MTG within the studied range of concentration.