Penicillin or cephalosporin antibiotics represented by following formula: ##STR1## where ##STR2## is an acyl group derived from an .alpha.-amino acid, N-ammonium salt of .alpha.-amino acid, or N--(C.sub.1 to C.sub.10) acyl-.alpha.-amino acid; COOM is carboxy or carboxylate salt group!
Are prepared by treating an amino compound selected from a group consisting of 6-aminopenicillanic acid, 7-aminocephalosporanic acid, 7-aminodeacetoxycephalosporanic acid, and their salts with an ester represented by following formula: [equation included]
where ##STR3## is as defined above and R.sup.1 is a (C.sub.1 to C.sub.10) alkyl group! BY ENZYMATIC ACYLATION EFFECTED WITH MYCELIUM OR MYCELIUM PREPARATION FROM A MICROORGANISM BELONGING TO GENUS Aphanocladium or Cephalosporium.