<b>Abstract</b>— <p>Fatty acid ethanolamides exhibit a variety of biological effects. Recently, it has been shown that diethanolamides of palmitic and stearic acids are capable of suppressing the growth of tumor cells. At the same time, these substances are characterized by low solubility in aqueous environments, which necessitates the search for ways to increase their solubility and biological activity. One approach to solving this problem could be the use of analogues of natural metabolites of fatty acid ethanolamides as solubilizers: diethanolammonium salts of fatty acids, which are ionogenic surfactants. The aim of this work is to study the effect of diethanolammonium salts of fatty acids on the solubilization and antitumor effects of palmitoyl and stearoyl diethanolamides. In experiments in vitro, palmitoyl and stearoyl diethanolamides have been shown to inhibit cell growth of C6 and HeLa glioma tumor lines. Diethanolammonium salts of caprylic, capric, lauric, palmitic, and stearic acids have a weak effect on cell growth; however, at the same time, they enhance the inhibitory effect of palmitoyldiethanolamide by 1.1–10.9 times and stearoyldiethanolamide by 1.1–8.3 times. The severity of the inhibitory effects of palmitoyl and stearoyl diethanolamides and their compositions with diethanolammonium salts of fatty acids depends on the dose of the drugs and the cell type. In relation to normal rat brain cells, the effects of the drugs are manifested to a lesser extent compared to the effects on rat C6 glioma tumor cells. In general, the ability of diethanolammonium salts of fatty acids to promote solubilization and enhance the antitumor efficacy of diethanolamides of palmitic and stearic acids has been demonstrated in vitro.</p>