C3PA (factor B) concentrations taken as an indication of alternate pathway development for neonates and adults were compared. The mean level for umbilical cord sera was 39 +/- 2%, with a range of 19.5-77.5%. The normal adult mean level was 74 +/- 4%, with a range of 43-108%. The difference between the two is highly significant (P less than 0.001). The ration of neonatal C3PA to adult C3PA is 0.52 +/- 0.10. In only one case was the newborn level greater than the mean adult value. There is positive correlation, r = 0.18, with gestational age, although it falls short of statistical significance (P greater than 0.1). There were no differences between the male and female neonates. C3PA titers were compared with C3 concentrations and so plotted. Although there was a positive correlation, r = 0.22, it was not statistically significant (P = 0.1). In an infant with gram-negative septicemia, the C3PA concentrations were much greater than the mean value found in normal cord sera. They were also greater than the mean value for normal adult C3PA titers, the multiple being 1.8-2.5. On first determination, after 2 days of normal to slightly elevated temperatures, a value of 132 +/- 6% was found. The second determination with a spike to 101.5 degrees F, and gave the highest of the three titers, 185 +/- 4%. At the same time that the C3PA levels reached this peak, the fever dropped to normal. At the time of the last determination, the C3PA levels had returned to that of the original sample, 125 +/- 4%. This study demonstrates that the cord sera of the normal term neonate is deficient in C3 and C3PA when compared with adult controls. Neither C3 nor C3PA correlated with gestional age. C3PA levels increase steadily as C3 titers increase and comparable ratios to adult values indicate that the alternate pathway is probably maturing at the same rate as the classic pathway. The results in the septic infant may represent a response to an inflammatory condition (acute phase phenomena), a block in alternate pathway expression, or synthesis beyond increased C3PA catabolism.