Imipenem (IMI) forming round cells in Gram negative rods reduces in subinhibitory concentrations (subMIC) the seroresistance of E. coli. This effect is distinctly more pronounced in a moderately seroresistant strain of E. coli than in a high seroresistant one. Conversely, human serum (HS) increases the sensitivity of E. coli strains to IMI dependent on their original seroresistance. In contrast, ampicillin (AMP), a filament inducer in E. coli, reduces equally seroresistance but only to a minimal degree and that only in a moderately seroresistant strain; the high seroresistant strain was nonreactive in this respect. It was concluded, that a synergism of antibiotics and bactericidal serum effect is predominantly produced with round cell forming antibiotics, whereas filament forming ones show only minimal effects. Moreover, the original seroresistance of strains in apparently important for the degree tho which these phenomena are expressed.