1. In an endemic area of Q fever in south-east England, two flocks of sheep, with serological evidence of infection by Rickettsia burneti, were examined for excretion of the rickettsia at parturition.
2. R. burneti was isolated from two out of ninety-six placentas collected from 118 ewes which lambed in one flock. The organism was also isolated from ‘wool tags’ obtained from one of the ewes which voided an infected placenta.
3. Infectivity titrations showed that the two placentas contained respectively 104·5 and 101·5 guinea-pig infective doses per gram of cotyledon. The wool tag from the sheep which voided the more highly infected placenta contained 103·25 infective doses per gram.
4. Except for one doubtful result, which could not be confirmed, no isolations were made from 194 placentas collected from 199 ewes in the other flock.
Details of the design of the microblender were supplied by Dr A. P. Goffe and Mr T. Nash of the Central Public Health Laboratory, Colindale, to whom we are much indebted. A gift of Triton X-100 was kindly given to us by the Rohm and Haas Corporation, Philadelphia 5 Pa.