Forty-one pregnant sheep showing positive immune responses to Brucella melitensis in serological or allergic tests were selected from naturally infected flocks and kept in an isolated pen for lambing. The resulting 62 lambs were maintained in the same pen with their dams during lactation. When the lambs were weaned, the dams were slaughtered for bacteriological study and the lambs were reared in a clean pen. Fourteen ewes excreted B melitensis during lactation and 17 were found to be infected postmortem. B melitensis was not isolated from seven lambs (three born to infected dams) which died after birth or from eight seronegative lambs (four born to infected dams) which were slaughtered between two and seven months after weaning. However, one permanently seropositive lamb born to a culture-negative dam was found to be infected when necropsied five months after weaning. The remaining 46 lambs were reared until adulthood and slaughtered at intervals for bacteriological study. Four ewe lambs (two born to culture-negative dams) were found to be infected postmortem, but were negative in immunological tests for B melitensis.