The gibberellin (GA) 2fl-hydroxylases in mature and immature seeds of Pisum sativum have been partially purified and characterised. The enzymes are unstable when stored below pH 7.0 or in the absence of a thiol reagent. The optimum assay pH is between 7.4 and 7.8 and activity is dependent upon the presence of ~-ketoglutarate, Fe z + and ascorbate. The 2fl-hydroxylase activities for GA1, GA4, GA9 and GA2o are chromatographically inseparable and correspond to a protein of Mr 44 000. The rate of GA 2fl-hydroxylation varies according to substrate and some evidence indicates that the 2fi-hydroxylase activities for GA1 and GA~ and for GA9. and GA2o may reside in different proteins. During pea seed maturation, the specific activity of the enzyme(s) increases dramatically and reaches a maximum at a time when endogenous GAg, GA/o, GA29 and GAs 1 are also at their greatest concentration. This correlation is not the result of substrate induction of enzyme activity. Since the GA 2fl-hydroxylases operate at maximal rate at low substrate concentrations they are incapable of rapidly 2fl-hydroxylating excessive quantities of (exogenously applied) GA1 or GA2o. On the basis of the kinetic parameters of the GA 2fl-hydroxylase activities, a generalised model is discussed for the control of the steady-state levels of bioactive hormone under normal physiological conditions.