Mutations in the ftsA gene of Escherichia coli conferred a higher resistance to lysis induced by penicillin or by a combination of cefsulodin and furazlocillin. The ftsA2 allele codes for an FtsA protein which is inactive at 42 degrees C but is able to regain its activity once it is transferred back to 30 degrees C; ftsA2 filaments formed at 42 degrees C in the presence of penicillin divided once the penicillin was removed and the temperature was lowered to 30 degrees C. Potential septation sites in the filaments of wild-type cells treated in the same way remained inactive. The binding of a radioactively labeled derivative of ampicillin to penicillin-binding protein 3 (PBP3) was significantly decreased in strain D-3, containing the mutant allele ftsA3, when the binding assay was performed at the restrictive temperature. A molecular species able to cross-react with an anti-PBP3 serum was nevertheless found to be present in the envelope of D-3 cells. These observations suggested that the FtsA protein, a protein with a structural and regulatory role in septation, and PBP3, a protein enzymatically active in the synthesis of murein for septation, interact with each other.