The Amt-1 protein from Archaeoglobus fulgidus is a member of the ubiquitous Amt family of ammonium transport proteins. Using an ammonium uptake assay based on the modified Nessler reaction we were able to measure the activity of different variants (mutants) of Amt-1 in vivo. In this context, we examined the functional role of Amt-1 in ammonium transport, in the light of conflicting proposals with regard to both the mode of action of Amt proteins and their substrate, i.e. ammonia or ammonium. Our preliminary functional results pertaining to different Amt-1 variants demonstrated that they are all capable of ammonium uptake, but the rate of uptake varies among different variants.