摘要：An experiment to determine Thai catfish's (Pangasius hypophthalmus Fowler) immunity was carried out using different levels of ascorbic acid (0, 1.000, 2.000 and 3.000 mg/kg feed). Fish of 15-20 g in weight were kept in aquaria for 6 weeks with density of 15 fish/aquaria. Feed was given at 5-10 % of total biomass with frequency of three times a day. The blood sampling was taken every week and the challenge test with Aeromonas hydrophila (106cells/mm3) injection intramuscular was done on the 30th day. The result of this experiment shown that feed with ascorbic acid of 2.000 mg/kg was elevated the cellular responses such as: leucocyte total (34.850 cels/mm3), differential of leucocyte (lymphocyte: 72,2%, monocyte: 8,0%, neutrophyl: 7,8%), phagocytic index (13%) and humoral response (titre antibody: 0.829 serum aglutination unit), which at the same time proves high level of survival rate against the artificial injection using A. hydrophila. Key words : Ascorbic acid, fish immunity, Thai catfish, Pangasius hypophthalmus. ABSTRAK Suatu penelitian telah dilakukan di laboratorium dengan menggunakan ikan jambal Siam (Pangasius hypophthalmus Fowler) untuk melihat tingkat kekebalan ikan dengan menambahkan vitamin C pada pakan (0, 1.000, 2.000 dan 3.000 mg/kg pakan). Ikan jambal Siam ukuran 15-20 g dipelihara dalam aquarium selama 6 minggu dengan kepadatan 15 ekor/wadah. Pemberian pakan dilakukan 3 kali sehari sebanyak 5-10% dari bobot biomasa, pengambilan contoh darah dilakukan setiap minggu dan uji tantang dilakukan pada hari ke-30 dengan bakteri Aeromonas hydrophila (106 sel/mm3) secara intramuskular. Hasil penelitian menunjukkan bahwa penambahan vitamin C sebanyak 2.000 mg/kg pakan menyebabkan meningkatnya respon seluler antara lain: total lekosit (34.850 sel/mm3), jenis lekosit (limfosit: 72,2%, monosit: 8,0%, netrofil: 7,7% dan trombosit: 17,6%) indeks fagositik 13% dan respon humoral (titer antibodi: 0,829 unit serum aglutinasi). Kelangsungan hidup ikan jambal Siam dengan respon seluler demikian mencapai 100%. Kata kunci : Vitamin C, imunitas ikan, ikan jambal Siam, Pangasius hypophthalmus.
