Human surfactant protein A (hSP-A), a molecule of innate immunity and surfactant-related functions, consists of two functional genes, SP-A1 and SP-A2. SP-A expression is regulated by several factors including environmental stressors. SP-A1 and SP-A2 5'UTR splice variants have a differential impact on translation efficiency and mRNA stability. To study whether these variants mediate internal ribosome entry site (IRES) activity (i.e. cap-independent translation) we performed transient transfection experiments in H441 cells of constructs containing one SP-A1 (A'D', AB'D', or A'CD') or SP-A2 (ABD) 5'UTR splice variant between the Renilla and firefly luciferase genes of a bicistronic reporter vector. We found, a) variants A'D', ABD, AB'D' exhibit significantly higher IRES activities than negative control (no SP-A 5'UTR), and A'CD' has no activity. The order of highest IRES activity was ABD>A'D'>AB'D. b) IRES activity of ABD significantly increased in response to diesel particulate matter (20 µg/ml) but not in response to ozone (1 ppm for 1 hr); c) Deletion mutants of ABD revealed regulatory elements associated with IRES activity. One at the end of exon A attenuated activity, whereas a region containing a short adenosine-rich motif in the second half of exon B and the start of exon D enhanced activity; d) Elimination of a predicted double-loop structure or increase in free energy significantly reduced IRES activity. e) Elimination of one or both double-loop structures in A'D' did not affect cap-dependent translation activity. Thus several factors, including cis elements and secondary structure type and stability, are required for hSP-A 5'-UTR variant-mediated cap-independent translation.