The presence of interleukin-15 (IL-15) mRNA in human placenta has been demonstrated previously. The present study was undertaken to investigate the expression profiles of IL-15 mRNA and protein in early and late gestational placental tissues, and also the effect of labour on its production. Levels of placental IL-15 expression were also determined in patients presenting with pre-eclampsia. An explant culture system was used to study the release of immunoreactive IL-15 by the placental tissues. Enzyme-linked immunosorbent assays were employed to quantify concentrations in the culture medium. The results showed that placental tissues from all groups released immunoreactive IL-15 into the culture medium. Moreover, the level of secretion by the term placental tissues was much higher than that by first trimester tissues. The presence of labour at term resulted in a further increase in placental IL-15 production. Reverse transcription-polymerase chain reaction (RT-PCR) was used to demonstrate the expression of IL-15 mRNA in these tissues. The results confirmed the expression of IL-15 in placenta from all the groups and the mRNA levels in the samples was highly correlated with the respective protein concentrations. Levels of both IL-15 mRNA and protein were significantly reduced in the pre-eclamptic placental tissue compared with the normal controls. The present study suggests an important role for this novel cytokine in human pregnancy.