Fluorinated derivatives of erythromycins A, B, C and D, having antibiotic activity, are prepared, starting from 8,9-anhydroerythromycin 6,9-hemiacetals or their N-oxides, through the reaction with a compound capable of generating electrophilic reactive fluorine, the resulting reaction product undergoing thereafter a reduction, possibly together with a methylation, to the corresponding (8S)-8-fluoroerythomycins.