Degradation　of　native　κ-carrageenan　was　performed　using　acid　hydrolysis　aided　with　microwave　heating.　Combined　with　nonofiltration　membrane　(cut-off　molecular　weight　250　Da)　separation,　1.　400　Da　-　50　kDa　low-molecular-weight　(LMW)　κ-carrageenans　were　obtained.　Narrow　molecular　weight　distribution　of　LMW　κ-carrageenans　could　be　prepared　under　pH　2.18　during　the　microwave　power　range　investigated.　The　in　vivo　anti-influenza　virus　(IV)　activity　of　three　kinds　of　LMW　κ-carrageenans　(3,　5,　and　10　kDa),　their　acetylated　derivatives　(acetylation　degree　of　1.5),　as　well　as　an　acetylated　and　sulfated　derivative　of　3　kDa　carrageenan　(acetylation　degree　of　1.0　and　sulfation　degree　of　2.4),　were　investigated　using　FM1-induced　pulmonary　oedema　model.　These　LMW　κ-carrageenans　showed　significant　inhibition　against　FM1-induced　pulmonary　oedema　as　compared　with　the　virus　control,　although　their　activities　were　inferior　to　that　of　positive　control,　Rabivirin.　Introduction　of　acetyl　groups　greatly　increased　their　anti-IV　activity.　The　acetylated　3-kDa　κ-carrageenan　exhibited　comparative　activity　with　Rabivirin　at　both　doses　of　6　and　30　2.　mg/kg　·d,　and　the　acetylated　and　sulfated　derivative　of　3　kDa　carrageenan　displayed　higher　activity　than　Rabivirin　at　the　dose　of　30　mg/kg·d.　These　results　disclosed　that　3　kDa　κ-carrageenan　with　proper　acetylation　degree　and　sulfation　degree　was　a　potential　candidate　against　influenza　virus.　©　2012　Wiley　Periodicals,　Inc.