Abstract: The present inventors have shown that MASP-depleted MBL is able to recruit MASPs from plasma and successfully activate complement cascade. Furthermore, it has been discovered that MBL purified as a complex has limited ability to activate the complement cascade when compared to MASP-depleted MBL. Accordingly, the present invention provides a pharmaceutical composition comprising an isolated non-recombinant mannose binding lectin (MBL) substantially free from activated MBL associated serine proteases (MASPs) together with a pharmaceutically acceptable carrier or diluent. Also provided is a method of treating a subject in need of MBL comprising administering to the subject an effective amount of a pharmaceutical composition of the invention.