Haemolytic assays were used to compare alternative and classical complement (C) pathway activities in sera obtained from neonatal pigs reared on porcine colostrum, bovine colostrum or an immunoglobulin-free synthetic diet. Dramatic increases in immunoglobulin concentrations were noted in the colostrum-fed animals during the first day of life, but there was not a concurrent, marked increase in either classical or alternative C pathway activity. Whether fed on homologous or heterologous colostrum, neonatal pigs had a similar gradual increase in alternative and classical C pathway activity in the post-natal period. If direct passive absorption of C components occurs in newborn pigs, it has only a minor influence on functional levels of alternative and classical C pathway activity in their sera. In pigs fed homologous and heterologous colostrum there was, respectively, an 83% and 80% increase in classical pathway activity, but only a 13% and 12% increase in alternative pathway activity during the first 3 days of life. Pigs fed the immunoglobulin-free synthetic diet had a 37% increase in classical C and a 24% increase in alternative C pathway activity. Part of the increase in classical C pathway activity in the post-natal period may be caused by a stimulating factor in colostrum. Most if not all of the increase in alternative C pathway activity and some of the increase in classical C pathway activity is most likely caused by normal humoral homeostatic mechanisms in the neonatal pig.