The present invention concerns a method for production of an active ingredientof a drugor diagnostic agent, in which(a) MDCK cells are infected with a virus and(b) the MDCK cells are cultured in suspension culture on a commercial scaleunderconditions that permit multiplication of the virusesin which culturing occurs in a volume of at least 30 L.The invention also concerns a method for production of a drug or diagnosticagent inwhich an active ingredient is produced according to the above method and mixedwith anappropriate adjuvant, auxiliary, buffer, diluent or drug carrier.