The cells of line L originated from the connective and adipose tissue of mice C3H were cultivated in nutrient medium with a 60% bovine serum. Some cells differentiated into adipose cells with formation of structures similar to normal adipose tissue. The occurring differentiation was evaluated by the formation of characteristic signetring cells whose cytoplasm was filled with neutral fat with a positive reaction for lipids in staining with a mixture of sudan III and sudan IV. The conclusion was drawn that prolonged existence of cells outside the organism (in vitro cultivation) and intracellular changes (accompanying the transformation), including those of mutation character, did not deprive the cells of their ability to differentiate.