Magnus Doverskog has been working as a Ph.D. student at the department of Biotechnology, Royal Institute of Technology, Stockholm, from January 1995 to the dissertation, May 26, 2000, under my supervision. From almost daily contacts during five years, I know Magnus Doverskog very well. Dr. Doverskog has been working on physiology and metabolism of cultured insect cells. Part of the work has been focused on the central metabolism, using NMR techniques for tracing 15N and 13C in metabolites. This work lead to a new view on the metabolism in Spodoptera frugiperda, Sf9, insect cells. Most surprisingly, it was shown that the nitrogen fixation system is present in these cells. This system was earlier thought to be present only in microorganisms and plants. Another part of Dr. Doverskog's work has dealt with amino acid metabolism, and in particular that of cystine/cysteine. All cultured insect cells had been reported to require cystine in the medium for growth. Dr. Doverskog could show that Sf9 cells were able to synthesise cysteine via the cystathionine pathway, provided that the cells were in a particular physiological state. This work lead to an investigation of the occurrence of autocrine growth factors and hormone systems in Sf9 cells. An insulin binding protein was identified in culture supernatants of Sf9 cells. This was the first report of such a protein in insect cells. A small peptide required for initiation of mitosis was also detected in the culture supernatant. In addition, the loss of productivity occurring at high cell densities in the baculovirus expression system could be related to auto-regulatory events controlling cell cycle progression. These findings will have a great impact for developing new control strategies for the baculovirus expression system. The papers included in the thesis are of high quality. Magnus Doverskog's Ph.D. work covers a broad range of subjects and methods. He has carried out his tasks with great enthusiasm and skill.