Small doses of Escherichia coli endotoxin or of viable E. coli caused only a mild illness in pregnant mice but severely impaired fetal development. After intravenous injection and after experimental renal infection E. coli multiplied in placental tissue, causing resorption of the conceptuses and sometimes fetal infection. It is suggested that in humans coliform bacteraemia sometimes results in abortion and premature delivery because of the placental damage brought about by E. coli and its abortifacient endotoxin. Isolates of group B streptococci from various sites in 175 patients varied markedly in their pathogenicity for mice, and pregnant animals were less susceptible to infection than non-pregnant. In late pregnancy streptococci multiplied rapidly in placental tissue, suggesting that the placenta may be a significant focus of infection at the time of delivery. After antibiotic treatment the majority of mice infected with a highly pathogenic strain died but the majority of those infected with a strain of low pathogenicity survived. Preliminary experiments suggest that guinea-pigs and rabbits may be useful for studying the role of maternally derived antibodies in protecting the neonate against group B streptococci.