Methods for the synthesis of 19-hydroxy and 19-oxo derivatives of 16α-hydroxytestosterone [(11 b) and (12b) respectively] have been developed. These compounds were labelled with ¹⁸O, ²H, and ³H at C-19 and also with ³H at C-17. The conversion of [17α-³H]-16α,19-dihydroxytestosterone (11c) into oestriol (4c), using human placental aromatase was demonstrated in good yield and it was shown that in this process the 19-oxo compound (3c) is involved as an intermediate. The use of 16α,19-hydroxytestosterone, labelled with ³H predominantly in the H_Si position, led to the conclusion that in oestriol biosynthesis the step, –CH₂OH→–CHO, is accompanied by the loss of H_Re and in the overall process the C-19 is ejected as HCOOH. On conducting experiments with either ¹⁸O₂ or substrate containing ¹⁸O at C-19 it was shown that, in the conversion of 16α-hydroxy-19-oxotestosterone into oestriol, an atom of oxygen from O₂ is incorporated into the formate. These features are similar to those already established for the corresponding biosynthesis of oestrone/oestradiol from androstenedione/testosterone. Our previous postulate that in oestrogen biosynthesis the same enzyme is involved in the hydroxylation reaction, –CH₃→–CH₂OH, and in the conversion, –CH₂OH→–CHO, as well as in the final cleavage of the C-10–C-19 bond is further developed. Attention is drawn to the fact that, if cytochrome P-450 dependent reactions are viewed to occur via a radical mechanism, then a concept can be developed which unifies the wide variety of transformations catalysed by this group of enzymes. The diversity of reactions would then arise from the alternative mode of decomposition of radical species by one or a combination of the following processes: (a) hydrogen abstraction; (b) disproportionation; (c) fragmentation; and (d) association of radicals.