Biocompatible,　near-infrared　luminescent　gold　nanoclusters　were　synthesized　in　situ　using　as-prepared　chitosan　grafted　with　N-acetyl-l-cysteine　(NAC-CS).　The　fluorescent　gold　nanoclusters　coated　with　chitosan-N-acetyl-l-cysteine　(AuNCs@NAC-CS)　were　aggregated　by　multiple　ultrasmall　gold　nanoclusters　closing　with　each　other,　with　strong　fluorescence　emission　at　680　nm　upon　excitation　at　360　nm.　AuNCs@NAC-CS　did　not　display　any　appreciable　cytotoxicity　on　cells　even　at　a　concentration　of　1.0　mg　mL-1.　AuNCs@NAC-CS　were　more　insensitive　to　H2O2　and　trypsin　compared　with　fluorescent　gold　nanoclusters　coated　with　Albumin　Bovine　V　(AuNCs@BSA),　which　make　them　have　long　time　imaging　in　HeLa　cells.　Furthermore,　the　obvious　fluorescence　signal　of　AuNCs@NAC-CS　appeared　in　the　liver　and　kidney　of　the　normal　mice　after　6　h　injection.　And　the　fluorescence　intensity　decreased　after　that　because　of　the　highly　efficient　clearance　characteristics　of　ultrasmall　nanoparticles.　These　findings　demonstrated　that　AuNCs@NAC-CS　possessed　good　fluorescence,　low　cytotoxicity,　and　low　sensitivity　to　some　content　of　cells,　allowing　imaging　of　the　living　cells.　©　2018　American　Chemical　Society.