ABSTRACT OF THE DISCLOSURE   The present invention relates to new cephalosporins and methods for their preparation.  The invention also relates to the preparation of pharma-ceutical preparations containing the cephalosporins and to methods for the pharmacological use of the cephalosporins.         In particular this invention relates to a process for the preparation of compounds of the formula                 I     or a pharmaceutically acceptable salt thereof, in which formula R1 is selected from the group consisting of a phenyl group, a thienyl group, a furyl group, a pyridinal group, and a tetrazolyl group, R2 is selected from the group consisting of a group S-R4 where R4 is an imidazolyl, benzoimidazolyl, benzothiazolyl, benzoxazolyl, thiadiazolyl or 5-methylthiadiazolyl group; a xanthate group; a dithiocarbamate group, an azido group; a hydrogen atom; an acetoxy group 1-pyridinium; R3 is selected from the group consisting of     wherein R5 is a hydrogen atom or a methyl group; R7 is a lower alkyl group, a cycloalkyl group of 3 to 7 carbon atoms; a phenyl or naphthyl group; an aralkyl group such as benzyl or naphthylmethyl; and wherein the alkyl, cycloalkyl, aryl and aralkyl groups may be substituted with one or more groups selected from the class consisting of amino groups, methyl-amino, diethylamino and acetamido groups, fluorine, chlorine or bromine atoms, nitro groups, lower alkoxy groups, X1 is selected from the group consisting of O and S; Y1 is selected from the group consisting of a hydrogen atom; a hydroxy group; a bromine, chlorine, fluorine or iodine atom; an azido group; an amino group; a substituted amino group selected from methylamino, diethylamino, benzyl sulphenylamino, methoxy-(hydroxy)phosphinylamino and acetamido; n is 0 or 1, m is 0 or 1; characterized in a)        reacting a compound of the formula   or a salt thereof with a compound of the formula R3' - Y2 to form a compound of the formula   in which formulae X1, R3, m and n have the meanings given above, Y2 is a halogen group or a functionally equivalent group thereof such as an organic sulphonic acid residue and R1', R2', and Y2' are R1, R2 and Y1 respectively as defined above or in case R1, R2 and Y1 contain amino or carboxy groups, protected derivatives of R1, R2 and Y1, whereafter, if necessary, the protecting groups are removed to give a compound of the formula I; or b)        reacting a compound of the formula   with a compound of the formula   to form a compound of the formula   in which formulae R1', X1, Y1', R2', R3', m and n are as defined above and -CO-Z is a reactive group capable of reacting with an amino group under formation of an amide moiety, whereafter, if necessary, the protecting groups are removed to give a compound of the formula 1; or c)         oxidizing a compound of the formula   to its corresponding sulphoxide and thereafter treating this with an acid catalyst to form a compound of the formula   which is then, i? desired or necessary, converted to any one of the compounds of formula I; or d)         reacting an imino-ether of cephalosporin with a compound of the formula   wherein R1', X1, Y1', m, n and Z have the meanings given above and then hydrolyzing the thus formed compound to form a compound of the formula   whereafter the protecting groups, if necessary, are removed to give a compound of the formula I, which, if desired, is converted to a therapeutically acceptable salt thereof.  The compounds of this invention are of value in the treatment of infectious diseases in man and animals caused by bacterial organisms.  The compounds of this invention are prepared according to known methods for the preparation of cephalosporin derivatives.