Preterm births occur in 12 percent of pregnancies. Preterm infants are at risk for long term health problems such as impaired hearing and vision, cerebral palsy and developmental delays. This study will characterize the human maternal preterm birth immune system using Cytometry-Time-of-Flight analysis of whole blood. A mass cytometry assay was implemented in a cross-sectional study of 19 women with a history of term or preterm birth to determine whether immune traits in peripheral blood differentiate the two groups in the absence of pregnancy. Twentyseven phenotypic and 11 intracellular markers were simultaneously analyzed in whole blood samples stimulated with lipopolysaccharide (LPS at 0, 0.1, 1, 10, and 100 ng mL21) to examine dose-dependent signaling responses within the toll-like receptor 4 (TLR4) pathway. Whole blood samples were collected from non-pregnant women with a history of term (n = 10) or preterm (n = 9) birth.