Influenza B viruses are a significant cause of disease and influenza B antigens are present in all human vaccines. Achieving suitable yields of seed viruses is often difficult for vaccine manufacturers. With influenza A viruses increases in yields have been achieved by the preparation of reassortants between a high-yielding donor strain and an epidemic strain. However, reassortment of influenza B viruses for the preparation of seeds has not been usually undertaken due to the lack suitable donor strains. Such an approach, which formed the basis of this thesis, could improve vaccine yields, lower costs and introduce a further element of predictability to vaccine manufacture. Potential donor strains were prepared from B/Lee/40 (B/Lee) by two approaches involving the selection of stable cold- and high- temperature mutants. Initial passaging was undertaken in specific-pathogen-free (SPF) chicken embryo kidney (CEK) cultures and later passage in SPF embryonated chicken eggs. Both approaches were successful, although a smaller number of viable progeny could be isolated from plaques obtained at 38„aC. Potential donor strains, isolated by selection at either 25 or 38„aC and plaque-purified in SPF CEK cultures, were tested for haemagglutinin and infectious titre, in comparison with the original parental strain by three methods, and for differences in antigenicity by cross-haemagglutination-inhibition tests. Potential donor strains selected at temperatures of 25„aC (C25) and 38„aC (H38) produced haemagglutination titres of 320 units/50ƒÝL and infectivities of 8.57 and 8.39 50% egg infectious doses, respectively, when grown in eggs at the permissive temperature (34„aC). Reassorting experiments using the B/Lee-derived potential donor strains C25 and H38 and the epidemic strain, B/Johannesburg/5/99 (B/Johannesburg), showed that the preparation of reassortant progeny with both epidemic strain HA and NA was difficult. Only 1/24 of the resulting reassortants possessed both the HA and NA of the epidemic strain. None of the reassortant progeny produced in reassorting experiments using C25 and H38 and the epidemic strain B/Panama/45/90 (B/Panama) possessed the desired 6:2 gene constellation (i.e. genes for the two surface antigens of the epidemic strain and the remainder from the donor strain). The infectious titre of selected progeny from the reassortment experiments were determined by three methods and compared with their respective epidemic parents. Yields of several influenza B epidemic strains and potential donor strains were measured after growth in Madin-Darby canine kidney (MDCK) cells prepared in serum-containing (SC) and animal- and human-derived protein-free (AHPF) media. Optimal multiplicities of infection were determined for B/Panama, B/Johannesburg and C25 in MDCK cultures grown in SC medium. A series of experiments were then undertaken to determine the maximum virus yields in MDCK cells grown in SC medium, followed by a further experiment using C25, B/Panama, B/Johannesburg, and selected reassortants after preparation in AHPF medium. Cell culture yields from 5/6 viruses grown in MDCK cells prepared in AHPF medium were higher than in cells prepared in SC medium and approached those obtained in eggs.