Plasma from eight newborns (4 pre-term and 4 full-term) with early-onset (< 72 h) sepsis and six apparently healthy controls was analyzed. The presence of spots identified as haptoglobin and serum amyloid A protein was the electrophoretic result most consistently associated with disease. Time course monitoring showed rises, peaks and declines of spot intensity as expected for acute-phase proteins induced by transient stimuli. Haptoglobin beta chains appear to be undersialated in pre-term newborns, whereas post-translational modifications of alpha chains and serum amyloid A protein are similar to those observed in adults. The undersialation of beta chain and occurrence of alpha chain phenotypes different from those found in maternal serum indicate that perinatal haptoglobin originates from neonatal synthesis.