The possibilities of 6-APA and 7-ADCA acylation by means of mixed anhydrides from ethyl chloroformate and cyclic carboxylic acids were investigated. The complexity of acylation course was observed, which depends upon kind of aminoderivative and structure of starting acid, respectively. In the case of mixed anhydrides formed from acid derivatives of which vicinal carbon atoms of ethylene bond built on into appropriate cyclic system were connected with methyl and carboxylic group (-C)CH3=C(COOH(-), it was found, that the main reaction products were: the suitable ureids and ethoxypenicillin or ethoxydesacetylcephalosporin. After treatment of 6-APA or 7-ADCA with different esters of chloroformic acid the corresponding alkoxy-, aralkoxy- or aryloxyderivatives were obtained. The MIC estamination showed, that only penicillin derivatives may be considered as interesting microbicides.