Background: Thrombocytopenia is common in sick preterm babies in the first day of life. Despite this, platelet production in thrombocytopenic preterm babies has rarely been assessed. Methods: To address this problem I have developed miniaturised assays to study circulating megakaryocyte (MK) progenitors (BFU-MK and CFU-MK), total cultured MK precursors and mature MK, by culturing mononuclear cells purified from 0.5-1ml of preterm peripheral blood. MK lineage colonies and cells are identified by a MK specific anti-glycoprotein IIb/IIIa antibody (CD61) by APAAP. Results: i) Normal values for these cells at birth were established in cord blood from healthy term and preterm babies. ii) Circulating BFU-MK/CFU-MK, total cultured MK precursors and mature MK were then prospectively studied in 63 preterm babies (gestational age 24-34 wks). iii) At birth 18 (69%) of the thromocytopenic babies were also neutropenic. Conclusion: These data indicate that the principal cause of the thrombocytopenia and neutropenia in the preterm babies studied was <I>reduced platelet</I> <I>and neutrophil production</I> occurring as a consequence of reduced numbers of MK and neutrophil progenitors respectively. Taken together these data suggest the haematological abnormalities characteristic of new-born born to mothers with PIH or with IUGR (thrombocytopenia, neutropenia and polycythaemia) are a consequence of dysregulation of <I>fetal haemopoiesis</I> occurring proximal to committed MK and neutrophil progenitors, most likely at the level of the primitive multipotent haemopoietic stem cell (CFU-GEMM). Finally the value of miniaturised MK progenitor and precursor assays in evaluating rare or unusual cases of neonatal thrombocytopenia has been demonstrated. In the three examples investigated this approach provided unique insights into the pathogenesis of the thromocytopenia in each case.