Genetic and enzymatic analyses were made with the purH mutants of Salmonella typhimurium. These mutants are purine auxotrophs which are deficient in the conversion of phosphoribosyl-aminoimidazolecarboxamide (AIC) to inosine-5'-monophosphate (IMP). Two steps are required for this process: phosphoribosyl-AIC transformylase (EC 2.1.2.3) and IMP cyclohydrolase (EC 3.5.4.10). Genetic analysis identified two complementation groups, I and II, and a third group of noncomplementing mutants (I-II). Mutations in gene I lead to complete loss of transformylase activity and no loss of cyclohydrolase activity if the mutation is of the missense type, but partial loss if it is of the chain-terminating type (nonsense or frameshift). Gene II mutants are all of the missense type and show normal transformylase activity but no cyclohydrolase activity. The noncomplementing mutants (I-II) are all of the chain-terminating type and are completely deficient in both activities. The results are explained and discussed in terms of subunit interactions of a stable enzyme complex.