A factor has been demonstrated in the serum of some patients with chronic Gram-negative infections which specifically blocks bactericidal activity against the infecting organism. Sera with this factor, that had been obtained from patients suffering from chronic Pseudomonas aeruginosa infections, were fractionated by Sephadex G-200 and DEAE-Sephadex column chromatography. Blocking activity was detected in the Sephadex G-200 '7S' peak and eluted from the DEAE column along with the serum IgG. Immunoelectrophoresis studies of this material along with pure IgG showed that the bactericidal blocking factor in these patients was IgG. The blocking factor was specific in its ability to protect bacteria, and could be absorbed from the serum by the particular bacterial strain isolated from the patient. Possible clinical importance of blocking activity by IgG is suggested by the persistent nature of the Ps. aeruginosa infections in these patients.