Rat kidney L-arginine:glycine amidinotransferase (transamidinase) has been purified previously to homogeneity as two fractions, designated α and β. No differences in the properties of these two fractions could be found. Two monoclonal antibodies (Tran/NS-1/1 and Tran/NS-1/3) to the purified α fraction of rat kidney transamidinase were produced, purified, and characterized. The results of competitive binding studies of the two monoclonal antibodies to α transamidinase were as follows: (1) Tran/NS-1/3 had no effect on 125I-Tran/NS-1/1 binding while Tran/NS-1/1 inhibited 125I-Tran/NS-1/1 binding; (2) Tran/NS-1/3 inhibited 125I-Tran/NS-1/3 binding while Tran/NS-1/1 had no effect on 125I-Tran/NS-1/3 binding. Therefore, Tran/NS-1/1 and Tran/NS-1/3 bound to different antigenic determinants on α transamidinase. 125I-Tran/NS-1/1 and 125I-Tran/NS-1/3 each had high avidity constants (~ 107-109) for both α and β rat kidney transamidinase. Tran/NS- 1/1 and Tran/NS-1/3 bound to human kidney transamidinase in ELISA assays. A quantitative immunosorbent inhibition assay for rat kidney transamidinase was developed with 125I-Tran/NS-1/3. Approximately 30 ng of immunoreactive transamidinase could be detected by this immunosorbent inhibition assay. The amount of Tran/NS-1/3 immunoreactive species in rat lung and testicular tissue by the immunosorbent inhibition assay correlated well with the amount of transamidinase activity found in those tissues. The availability of the monoclonal antibodies, Tran/NS-1/1 and Tran/NS-1/3, should facilitate studies of rat and human transamidinase structure and regulation.