The interferon (IFN)-α response of pigs to the stressing event of early weaning was investigated in a field trial. All the animals under study remained healthy and tested negative for common viral infections. However, a low-titered IFN-α response was detected in many sera by a bioassay on Madin-Darby bovine kidney (MDBK) cells on day +6 after weaning. Porcine IFN-α was unambiguously identified by a neutralization assay on a pool of IFN-α-positive sera. By gel filtration chromatography, the antiviral activity of sera on MDBK cells could be traced back to 3 components of apparent molecular mass 27/18/<14 kDa. Additional components of apparent molecular mass 58 and 41 kDa were revealed by ELISA in Nonidet P-40 lysates of peripheral blood mononuclear cells (PBMC). Also, many pigs tested positive in flow cytometry assays on PBMC for intracellular IFN-α. The expression of porcine IFN-α genes was investigated by reverse transcriptase (RT) real-time polymerase chain reaction at days -1, +6, and +12 with regard to weaning in PBMC of 9 piglets. On days -1 and +12, IFN A5, A6, A12, as well as (in fewer pigs) A1, A7, A11, and A2 genes were shown to be expressed. On the contrary, none of the above genes was expressed on day +6, when plenty of pig sera were IFN-α-positive. Our results indicate that weaning causes the release of IFN-α and the transient shut-off of the corresponding gene transcriptions in PBMC. Interestingly, only IFN A9 gene transcription was shown in vitro to be virus induction-dependent.