Abstract: Although cefathiamidine, one of the new semisynthetic antibiotics, has been used favorably in clinics, it is not efficacious enough for the control of gram-negative organisms. In order to find derivatives more active than the parent substance a series of new compounds, containing the substituted amidinothio group in the side chain at the C-7 position of cephalosporin and different heterocycles at the C-3 methylene position, has been synthesized. After preliminary screening it was found that compounds C-7906, C-7907, C-7911 and C-7924 showed high activity against gram-negative organisms as well as gram-positive organisms. Their activities are comparable with those of cefazolin.