Expression of GM-CSF mRNA is 4-fold lower in activated peripheral blood mononuclear cells (MNC) from newborn (Nb) compared to adult (Ad) (Cairo, et al, Pediatr Res, 30:362, 1991). While the GM-CSF transcription rate is similar in Nb and Ad MNC, transcript half-life is 3-fold less in Nb activated MNC. Inhibiting protein synthesis with 10μg/mL cycloheximide(CHX; 3h) after stimulation with 20ng/mL PMA + ≈2μg/mL PHA (3h) increased GM-CSF mRNA 3-fold in Nb MNC but had only a minimal effect in Ad MNC compared to PMA+PHA. To investigate the involvement of RNA binding proteins, such as the cloned AUF1 factor (Zhang, et al., Mol Cell Biol, 13:7652, 1993) interacting with 8 AUUUA-motifs (AU) in the 3′-untranslated region (8AU), cytoplasmic extracts were prepared from Nb vs. Ad MNC unstimulated or stimulated with PMA+PHA (6h), ±CHX (3h). Electrophoretic mobility shift assays using 32P-labeled 8AU RNA revealed two, RNaseT1-resistant, bound complexes from both Nb and Ad MNC extracts, and competitive assays localized the binding site of both complexes to the region containing 7 of 8 AU-motifs. One of the bound complexes was twice as abundant in Nb than in Ad MNC. Inclusion of AUF1 antiserum revealed a super-shifted complex at 30-fold higher levels in Nb than Ad MNC extracts. A human carcinoma cell line (5637) with extended GM-CSF mRNA half-life also showed very low levels of anti-AUF1 super-shifted complex. Anti-AUF1 immunoblotting showed considerably higher levels of the two smallest of four AUF1 protein species in Nb than in Ad MNC or 5637 extracts. Additionally,in vitro mRNA decay assays found the stability of 32P-8AU to be 85% less in Nb than Ad MNC extracts compared to transcripts lacking AU. These results suggest that destabilization of GM-CSF mRNA in Nb MNC is translation-dependent and that increased levels of specific AU-binding AUF1 species in Nb MNC may target transcripts for increased degradation which could account, in part, for dysregulation of Nb phagocytic immunity.