GO:0004643:   phosphoribosylaminoimidazolecarboxamide formyltransferase activity
Catalysis of the reaction: 10-formyltetrahydrofolate + 5"-phosphoribosyl-5-amino-4-imidazolecarboxamide = tetrahydrofolate + 5"-phosphoribosyl-5-formamido-4-imidazolecarboxamide.