We have used antibodies specific for troponin I proteins to examine human cardiac development and have detected a transiently expressed developmental isoform. This isoform is distinct from adult cardiac troponin I (TnIc) but is indistinguishable, on the basis of electrophoretic mobility and antibody reactivity, from the isoform found in slow skeletal muscle (TnIs). Furthermore, we show that mRNA for TnIs is present in fetal, but not adult, heart. Analysis of a developmental series of fetal samples indicates that there is a transition in expression from TnIs to TnIc which occurs between 20 weeks fetal and 9 months postnatal development.