Background: Complement is a crucial branch of both non-specific and specific immune system. C3 is the third complement component which plays a protective role in viral infections. There are two co-dominant inherited variants or allotypes for C3: C3 fast (C3F) and C3 slow (C3S). C3F variant has been linked to multiple diseases production including infections among liver transplantation recipients. Objectives: To investigate the most common risk factors for post-liver transplantation infections especially pre-operative colonization. To identify types and rates of post-liver transplantation infections and their causing organisms. To identify C3 allotypes for both liver transplantation donors and recipients, and to correlate recipients' allotypes with post-transplantation infections. Methodology: This study is a prospective study; it was conducted from January 2017 to August 2019 on 64 chronic cirrhotic patients, experienced liver transplantation in the National Liver Institute ICU, Menoufia, Egypt, and their donors. Blood cultures and other samples were collected according to site of infection using standard Microbiological sample collection methods and bacterial isolated were identified by standard microbiological methods using VITEK2 Compact automated ID/AST instrument. CMV IgG and IgM were detected by ELISA method. DNA was extracted and the extract was used for detection of C3S and C3F alleles by using Amplification Refractory Mutation System (ARMS). Results: The most common cause for liver transplantation was HCV related liver cirrhosis (26.56%). Risk factors for post-liver transplantation were Pre-operative microbial colonization (100%) and long operative time (9.875±2.45 h). 39.8% of post-transplantation infections occurred during second week post-operatively and the commonest infections were drain infections (29.5%) and urinary tract infection (27.3%). 51.7% of liver transplantation recipients were C3FF, 32.8% were C3FS and 15.6% were C3SS. C3FF recipients showed increased relative risk to develop CMV (5.2) and bacterial & fungal infections (2.2) than other recipients' allotypes. Conclusion: A comprehensive infectious diseases workup including detection of C3 allotype of the candidate for liver transplantation should be done pre-operatively to early detect and treat infections which can improve the outcome of the operation dramatically and improve patients' life style. More studies should be done to find out if there is relation between donors' and recipients' C3 allotypes as currently no clear data still present.