The total haemocyte count (THC), phenoloxidase activity, and respiratory burst were examined when the white shrimp Litopenaeus vannamei (10.42±1.39 g) were immersed in seawater (34‰) containing hot-water extract of brown alga Sargassum duplicatum at 100, 300 and 500 mg l−1, or injected with hot-water extract of S. duplicatum at 2, 6, 10 and 20 μg g−1.