Phagocytic activity of tilapia head kidney leucocytes at 4 days post-injection of P. vulgaris . Fish were I.P. injected with 0, 100, 300 and 500 μg/100 g of fish, and FCA. Phagocytic activity was measured on day 4 after injection. Data represent the mean + S.D. (n = 7). Statistical differences (p < 0.05) between groups are indicated by different letters over the bar.