Genes of the HOMC complex (Hox genes) are important controllers of structural polarity and cell fate in vertebrate development. Retinoic Acid(RA) plays a critical role in regulating these processes by directly activating the hox genes. The specificity of RA activity on the expression of the different hox genes is coordinated by temporal and dosedependent effects. Hoxa5 gene expression in fetal mouse lung has been previously reported by ourselves and others but little is known about the protein expression and the cellspecific regulation of gene expression. We hypothesized that Hoxa5 protein is expressed in the developing lung and that RA regulates cellspecific Hoxa5 expression. Western blot analysis showed that the Hoxa5 protein was expressed in the developing mouse lung from gestational day E13.5 to postnatal day P2 with a strong 33 kD band and a weaker 43 kD band, similar to previous reports of two Hoxa5 mRNA transcripts in fetal lung. Next, E17.5 fetal mouse lung fibroblasts were grown to confluence and treated with RA (108 104 M) for 3, 6, 14, 24, and 48 hours. Northern blot analysis showed increasing Hoxa5 mRNA expression with increasing RA dose and exposure time. Threefold increases at 24 hrs and 2.5fold increases at 48 hrs were seen for 105 and 104 M doses. We conclude that Hoxa5 protein is expressed throughout fetal lung development and that RA regulates Hoxa5 gene expression in fetal lung fibroblasts in a dose and timedependent manner. (Supported by HL 37930 and DK07471)