Thrombocytopenia is a common occurrence in sick newborn babies. Despite this, platelet production in the newborn has rarely been assessed, principally because of the difficulties of obtaining bone marrow, especially on a serial basis. We have developed two miniaturized assay systems to study megakaryocyte (MK) progenitor cell differentiation, from BFU-MK and CFU-MK to mature MK, by culturing mononuclear cells purified from 0.5-1 ml of neonatal peripheral blood. BFU-MK and CFU-MK were assayed in agar, whilst total cultured MK precursors and mature MK were assessed in liquid culture. In both systems, MK lineage cells were identified morphologically and by an anti-IIb/IIIa antibody (CD61). Normal ranges for MK precursors in term neonates were established from cord blood studies of 40 healthy term babies and compared with cord blood studies in 16 non-thrombocytopenic pre-term babies (gestational age range 24-36 weeks). Pre-term babies had greater numbers of all MK precursors than term babies: BFU-MK 414 +/- 61 v 151 +/- 18 colonies/ml (mean +/- SEM); CFU-MK 2444 +/- 337 v 869 +/- 64 colonies/ml; total MK precursors 213 +/- 36 v 54 +/- 6 x 10(3) cells/ml and mature MK 20 +/- 4 v 7 +/- 1 x 10(3) cells/ml. In addition, in newborn babies (n = 22), with no evidence of platelet consumption, circulating MK progenitor numbers at birth correlated with platelet numbers. These data indicate that in the newborn the measurement of circulating MK precursors provides a good indicator of megakaryocytopoiesis, and hence platelet production, and therefore is a useful and practical way of investigating neonatal thrombocytopenia.