During the past two decades, recombinant DNA technology brought about a significant increase in the number of protein drugs. The development of analytical techniques enables a high degree of clarification of the recombinant proteins, which in turn, brings about a significant decrease of the adverse effects. In spite of that, one of the capital problems, and challenges in the development of protein pharmaceuticals remains unsolved: the physical and chemical instability/degradation of proteins, which is in close connection with the appearance of toxicity through the stimulation of immune and/or allergic reactions, only to result with a decrease or a complete loss of the biological activity in the end. In this book we establish the HPLC methods for analysis of lenograstim and the products resulting from the chemical and physical instability of its molecule. These methods allow for the detection of the oxidized forms and aggregates of lenograstim at the very beginning of their formation that would signalize any changes in the stability of the molecule. The book would be of great value for scientists and practitioners working in research, development and production of protein drugs.