Inactive renin was partially purified from normal male plasma. It showed no enzymatic activity at pH 7.4, and its molecular weight by gel filtration was 53 000 compared with 45 000 for partially purified plasma active renin and 41 000 for pure renal renin. After exposure to pH 3.0 the inactive renin became enzymatically active but its molecular weight did not change. The acid-activation could be reversed when the renin was re-adjusted to pH 7.4 and warmed. Pure renal renin labelled with I125 was added to normal male plasma. It had a molecular weight of 40 000 by gel filtration. When the mixture was acidified and neutralized, some I125 label appeared in a high-molecular-weight peak, as might occur if the renin was associated with a binding protein. If the mixture of plasma and labelled renin was treated with guanidine hydrochloride, most of the label appeared in the high-molecular-weight peak. However, after both acid treatment and treatment with guanidine hydrochloride the 'high-molecular-weight' peak appeared in the void volume of the column (Mr 108 000) rather than in the position of inactive renin (Mr 53 000). Also, the I125 label in the peak was neither immunologically nor electrophoretically similar to renin. It may represent denatured renin bound to plasma proteins, e.g. alpha-2 macroglobulins. We conclude that 'inactive renin' is a prorenin-like material rather than a protein-bound form of active renin.