The relative amount of L-arginine:glycine amidinotransferase (transamidinase) protein in kidneys from rats fed a complete purified diet with and without the addition of creatine and/or glycine was determined by a monoclonal antibody-immunosorbent inhibition assay. Kidneys from the creatine-fed rats had 10% of the transamidinase activities 78% of the monoclonal antibody immunoreactive transamidinase protein as kidneys from the control rats. An excellent correlation between transamidinase activities and protein was reported previously when the amounts of enzyme protein were determined by immunotitration with polyclonal antibodies. One possible explanation for the contrasting results was that multiple forms of transamidinase are present in rat kidneys. If so, the monoclonal antibody may have recognized forms of the enzyme that were not decreased in amounts commensurate with the decrease in enzyme activities as a result of creatine feeding. Evidence is presented in this report that multiple forms of transamidinase are present in rat kidneys. The distribution of the isoelectric points of the individual forms of transamidinase in kidneys of the control rats appeared to be dissimilar from that in the creatine-fed rats. Therefore, an alteration in the distribution of the individual forms of the enzyme may be a factor in the alteration of transamidinase activities in creatine-fed rats.