Acetylene.reduction activity of detached soybean nodules was positively correlated with the nitrogen content of the host plants grown in nitrogen-free media. Acetylene reduction by nodulated root systems was related to light intensity preceding sampling; activity of detached nodules was lower and less influenced by light. Comparison of acetylene reduction and 15N 2 uptake by detached soybean nodules gave ratios of acetylene reduced to nitrogen fixed (C2~: NHs ratio) ranging from 2�7 to 4�2 in different experiments. The mean value for four experiments was 3�3�0�6 (S.E.). Different effects of oxygen were obtained with detached soybean nodules when 0�1 atm nitrogen and 0�1 atm acetylene were used as substrates. When the acetylene concentration was reduced to 0�005 atm the effects upon both assays were similar but the ratio of the two assays was still affected by p02.