In order to have a deeper insight into the retinol isomerization phenomenon, in this work different milk samples have been analysed for their content of trans retinol and its cis-isomers, by means of reliable HPLC techniques. Levels of the different isomers and the degree of retinol isomerization (13-cis/all-trans ratio, %) have been monitored during milk storage at different temperatures and after addition of specific microorganisms. In raw milk stored at 4°C for 96 h the degree of retinol isomerization shifted from 1·1 to 2·3%, while in raw milk stored at 22°C for 24 h it increased from 1·1 to 12·7%. Among microorganisms tested in pasteurized milk, the most active in causing an increment in the 13-cis/all-trans ratio (%), from 3·4 to 33·4% in 8 h, was Streptococcus thermophilus. The results obtained demonstrated a relationship between microbial evolution and retinol isomerization. Therefore, the determination of retinol isomers is of importance not only for a more precise evaluation of vitamin A activity but also for the evaluation of the microbiological quality of milk.