Background: Spontaneous bacterial peritonitis is very common bacterial infection in patients with cirrhosis and ascites, when first described its mortality exceeded 90% but it has been reduced to approximately 20% with early diagnosis and treatment. Mannose-binding lectin (MBL) is an important protein of humoral innate immune system with multiple carbohydrate recognition domain , it is able to bind to sugar group displayed on the surface of wide range of micro organism and therepy provide first line defense, it is also occur activate complement MBL produced in liver in response to infection and is a part of many other factor termed acute phase protein. We aimed to study the level of ascitic fluid mannose binding lectin in patients with spontaneous bacterial peritonitis. Methods: This study was conducted on 90 patients with cirrhotic ascites with and without spontaneous bacterial peritonitis admitted to Benha University Hospital and Manshyet El Bakriy Hospital. They were classified into two groups, group (I) included 45 cirrhotic patients with spontaneous bacterial peritonitis (35 males and 10 females), aged between 36 - 69 with a mean age of 52.96 ± 7.90 years while group (II) included 45 cirrhotic patients without spontaneous bacterial peritonitis (29 males and 16 females), aged between 28 - 65 with a mean age of 49.49 ± 10.21 years. Results: Regarding the ascetic level of MBL in patients with SBP, it was found in this study that the mean value of MBL level was significantly lower in SBP group than non SBP group. There are statistically significant positive correlations between MBL and Bilirubin, prothrombin time, INR and u MELD values and significant negative correlations between MBL and Platelets and serum ascetic albumin gradient values among patients with SBP. In the present study, the ROC curve shows that the best cut off point for MBL to diagnose cases with ascites with SPB cases was found 1307 with sensitivity 75.56%, specificity of 77.78% and area under curve (AUC) of 77.0%.  In this study, by multivariant analysis WBCs, RBCs, serum creatinine, ALT, serum albumin and MBL were significantly determining predictors for SBP positivity. Conclusion: The mean value of ascetic level of MBL was significantly lower in SBP group than non SBP group. Ascitic fluid MBL could be a good predictive and prognostic marker in patients with cirrhosis and spontaneous bacterial peritonitis.