The qualitative and quantitative composition of cytochromes in intact cells of Acinetobacter calcoaceticus and in particle fractions obtained from cells following ultrasonic treatment by differential centrifugation were studied using spectrophotometric methods. Acinetobacter calcoaceticus contains cytochrome b, cytochrome o and low amounts of cytochrome d. Both the absolute content of cytochrome b and o and the relative composition do not essentially vary with the carbon source used (hexadecane, acetate, succinate, malate, yeast extract). Only bacterial cultivated on yeast extract show, under simultaneous decrease of the content of cytochrome o, an increased formation of cytochrome d. In Acinetobacter, cytochromes appear not to be immediately involved in n-alkane hydroxylation.