This study examined the effect of inoculation with the plant-growth-promoting rhizobacterium (PGPR) Pseudomonas mendocina Palleroni, alone or in combination with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith) or Glomus mosseae (Nicol & Gerd.) Gerd. & Trappe, on antioxidant enzyme activities (superoxide dismutase, catalase and total peroxidase activities), phosphatase and nitrate reductase activities and solute accumulation in leaves of Lactuca sativa L. cv. Tafalla affected by three different levels of water stress. At moderate drought, bacterial inoculation and mycorrhizal inoculation with G. intraradices, alone or in combination, stimulated significantly nitrate reductase activity. At severe drought, fertilisation and P. mendocina inoculation, alone or in combination with either of the selected AM fungi, increased significantly phosphatase activity in lettuce roots and proline accumulation in leaves. Total peroxidase (POX) and catalase (CAT) activities increased in response to drought, whereas superoxide dismutase activity decreased. Inorganic fertilisation and both combined treatments of PGPR and AM fungus showed the highest values of leaf POX activity under severe drought. The highest CAT activity was recorded in the fertilised plants followed by the P. mendocina-inoculated plants grown under severe stress conditions. These results support the potential use of a PGPR as an inoculant to alleviate the oxidative damage produced under water stress.