C/EBP delta, a member of the leucine zipper transcription factor family, is expressed at higher levels in the lung than in any other tissue. We detected C/EBP delta mRNA and protein in NCI-H441 cells, a cell line derived from a human adenocarcinoma that produces surfactant protein A (SP-A). NCI-H441 cells were exposed to phosphorothioate-substituted antisense oligonucleotides directed against C/EBP delta. After exposure to the oligonucleotides, cells were harvested, total RNA prepared, and levels of mRNA for C/EBP delta, SP-A and a control, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), were quantified from Northern blots. An oligonucleotide that overlapped the translational start was effective in reducing C/EBP delta mRNA. Oligonucleotides that corresponded to regions upstream and downstream from the translational start were not as effective. The loss of C/EBP delta was accompanied by a decrease in the level of SP-A mRNA. The overlapping oligonucleotide was tested more extensively. After 72 h, antisense oligonucleotide at 3 and 5 microM reduced the level of C/EBP delta mRNA and protein by 50% or more as compared with sense and scrambled controls. The SP-A mRNA level was reduced even more, by about 75%. GAPDH mRNA was not affected. We conclude that C/EBP delta plays a role in the regulation of SP-A gene expression.