SummaryComplement activity was compared in 50 low birth weight infants divided into appropriate and small for gestational age groups; the influence of birth weight and gestational age on complement development was also investigated. CHW and kinetics (tHso) of both classical and alternative pathway activity of complement, C3, and Factor B levels were significantly higher in small for gestational age infants (classical pathway CHs, 630 HU/ml f 184 SD, C P tHs, 77 min + 47; aternative pathway CHS0, 44.8 HU/ml & 11.3; AP tHso, 56 min f 43; C3, 73.98 mg/dl f 12.68; and Factor B, 13.17 mg/dl 2 3.67) than in weight-matched appropriate for gestational age infants (CP CHW, 523 HU/ml 2 152; C P tH9, 105 min + 49; AP CHW, 38.8 HU/ml f 13; AP tHW, 90 min 2 53; C3, 58.14 mg/dl f 9.43; and Factor B, 9.32 mg/dl+ 1.73). Complement values were lower in low birth weight infants than in adult controls ( P < 0.001 in all cases). All complement parameters were mainly correlated with gestational age; CHso values of the classical and alternative pathways were also highly correlated with each other