1. A high meningococcus carrier-rate in a military unit denotes that dangerous overcrowding exists in this unit. 2. Severe overcrowding (i.e. when beds are less than 1 foot apart) is usually accompanied by a carrier-rate (serological) of at least 20 per cent. (Twenty per cent. is the danger line indicated in the War Office Memorandum on Cerebro-spinal Fever, March 1917.) 3. A carrier-rate of this height will usually imply that the mobilization standard of 40 square feet has been infringed, and also that beds are less than 1 foot apart. 4. A carrier-rate of 20 per cent. (without awaiting the occurrence of any actual cases of the disease) should be regarded as a signal for prompt and effective action to abolish overcrowding, and to improve ventilation, and to increase the distance between the beds to at least 2½ feet. 5. The distance between beds is of paramount importance. 6. Carrier-rates of between 10 and 20 per cent. are unsatisfactory and imply a certain degree of overcrowding. They must be watched with suspicion, and the mobilization standard strictly enforced. 7. Under the same conditions of overcrowding “non-contact” carrier-rates are the same as “contact” carrier-rates. 8. Quite a moderate degree of “spacing out” of beds, combined with simple methods for improving ventilation are highly efficient agents in reducing high carrier-rates. 9. When, however, a unit shows a high carrier-rate, insistence on the restoration of the mobilization standard is not sufficient. “Spacing out” must be carried further; and a distance of at least 2½ feet between beds insisted on. The “peace” standard of 3 feet between beds and 60 square feet of floor space with 600 cubic feet of air space would, of course, be still more effective. 10. The mobilization standard, introduced for a grave emergency, is the lowest possible concession to military necessity, which can be allowed with safety.