Abstract: There are disclosed a .beta.-lactam compound represented by the formula (I): ##STR1## wherein R.sub.1 and R.sub.2 are independently a hydrogen atom or a lower alkyl group; M is a hydrogen atom, a protective group or an eliminatable group which is easily hydrolyzable in a human body; R' and R" are independently a hydrogen atom or a protective group; A is a mercapto group substituted by a substituted or unsubstituted polycyclic nitrogen-containing heterocyclic ring or a group represented by the following formula (a):--OOCNR.sub.3 R.sub.4 (a)where R.sub.3 and R.sub.4 are independently a hydrogen atom or a lower alkyl group, provided that R.sub.1 and R.sub.2 are both hydrogen atoms, both R.sub.3 and R.sub.4 being hydrogen atoms are excluded,or its pharmaceutically acceptable salt, and a method for preparing the same , medicinal composition for microbism therapy containing the same and intermediates for synthesis of the same.