The influence of the gadolinium-containing drug Dipentast (0.5 M water solution of sodium gadopentetate) on the antineoplastic efficacy of X-ray radiotherapy has been studied. Dipentast possesses neither intrinsic antitumor effect, nor tumor-seeking capability. Mice C57Bl/6 with transplanted adenocarcinoma Ca755 were used in the study. The mice were irradiated with an X-ray machine with an anode voltage of 200 kV. The absorbed dose in the tumor after irradiation was 10 Gy for both experimental and control groups. The mice in the experimental group were treated with a single intraperitoneal injection of 0.3 ml of Dipentast. The observation of the tumour growth rate has shown that intraperitoneal administration of Dipentast prior to X-ray irradiation results in a significant tumour growth delay (12 ± 1 days for the experimental group versus 2 ± 1 in the control group), increasing the median life span from 36 ± 1 days in the control group to 43 ± 1 days in the experimental group, and leads to complete disappearance of the tumour in 25 ± 1% of the animals in the experimental group, while in the control group, no tumour regression was observed.