The present invention relates to a method of producing a fluorescent polypeptide complex characterized in that an isolated fluorescent polypeptide is inter-molecularly cross-linked with itself or one or more other polypeptides and wherein said complex is of 40 nm to 500 nm in size, it also relates to a complex obtainable by such method, to conjugates comprising such a fluorescent polypeptide complex and to the use of such complex or conjugate.