The passive protection of mice against an intracerebral infection with Bordetella pertussis, by antiserum introduced directly into the brain with the infecting organisms, was compared with the protection afforded by intraperitoneal antiserum. The antibody effective by the intracerebral route is that which is adsorbed onto the infecting organisms, although it does not affect the viability of the organisms in vitro in the absence of complement.

Passive protection against organisms introduced intracerebrally takes place in one of two ways, depending on the size of the challenge: (1) after 3–4 days' growth, the growth rate declines so that the number of organisms does not reach the figure lethal for the mouse; (2) the organisms do not appear to multiply, as their numbers decline from the moment of injection, so that the brain is sterile after 2 days.

Many of the mice protected against a challenge of 50,000 organisms (ca. 100 LD50) by intraperitoneal or intracerebral antiserum recover by the first mechanism. The second mechanism operates after a smaller challenge of 5000 organisms (ca. 10 LD50), irrespective of whether the antiserum is given intracerebrally with the challenge, or intraperitoneally within several hours of challenge. Too much antiserum given intracerebrally with a 50,000 challenge, but not with a 5000 challenge, inhibits protection.

In some mice, virulent antibody-treated organisms which have not been killed, grow very slowly over a long period, but are eventually eliminated from the brain.