SummaryLeukin, a cationic bactericidal protein of low molecular weight that is derived primarily from the polymorphonuclear leucocyte, was found to be present in amniotic fluid from 450 normal obstetric patients at teim. The concentration in the amniotic fluid was very similar to that found in circulating maternal plasma, probably indicating simple diffusion of leukin. Neutralisation of amniotic fluid leukin with specific antisera did not change thein vitrobactericidal activity of whole amniotic fluid. The role of leukin as a nonspecific host defence agent in amniotic fluid is thought to be minimal