Human colostrum and milk contain significant amounts of two soluble receptors (sTNF RI, sTNF RII) for TNFα. To determine whether these receptors are functional, we examined their ability to bind to and modify the bioactivity of recombinant human TNFα. Using a colostrum specimen containing 8.8 ng sTNF RI/ml (293 nM), the gel filtration chromatography - based molecular sizes of sTNF RI and sTNF RII increased sequentially when increasing amounts of rTNFα (0, 10, 100, 1000ng/ml representing 0, 0.76, 7.6 and 76-fold molar excesses of rTNFα) were added: sTNF RI: 49kDa→ 49kDa → 71kDa → 71kDa respectively; sTNF RII: 49kDa → 49kDa → 53kDa → 60kDa respectively. These shifts were consistent with molecular sizes reported by others and the known higher afinity of sTNF RI for TNFα. In the same experiments, rTNFα's size decreased from 49-51kDa at 7.6-fold molar excess to 42kDa at 76-fold molar excess. Using rTNFα covalently attached to beads, colostral sTNF RI was enriched 2900-fold (n=3) based on the ratio of sTNF-RI antigen to contained protein, but this material failed to re-bind to the rTNFα affinity matrix and was without consistent bioactivity in a murine fibrosarcoma cell (WEHI-13var) bioassay in which the LD50 was 10±6 pg rTNFα/ml (n=7). Two colostrum specimens (sTNF RI 8.8 ng/ml and 4.4 ng/ml) used at 1:16 final dilutions, shifted the LD50 4-fold and 12-fold (to 18 pg rTNFα/ml and 26 pg rTNFα/ml) respectively. These data demonstrate that soluble TNFα receptors in colostrum bind to rTNFα and can neutralize its bioactivity, and therefore, may contribute to the anti-inflammatory character of human colostrum and milk.