Azithromycin dihydrate is prepd. from azithromyin cby adding a base to aq. soln. of azithromycto crystallize the dihydrate, the aq. soln. having a pH of from 1 to 5 and contg. acetone. Azithromyc5 g was dissolved 22.6 mL water and 2.4 mL HCl (6 N) and to this soln. was added 25 mL acetone and 2.8 mL of 20 % aq. NaOH soln. to adjust the pH to 9.8. After stirring for 5 h at 20-25.degree., the suspension was cooled to 0-5.degree.. The resulting solid was collected by filtration, washed with water, and dried at 35-40.degree. to give 3.3 g azithromycdihydrate.