In the present investigation, microarray analysis was used to identify potential TCDD gene targets. Three microarray experiments were performed to study the effect of TCDD in an established chicken B-cell line (DT40), in a chicken macrophage cell line (HD11), and in the bursa of Fabricius from embryos exposed in ovo at 6 days of incubation. From the DT40 microarray analyses, clones with sequence similarity to the apoptotic genes caspase 8 and caspase 9, and the transcription factor NFΜB, among others, were identified. Real-time quantitative polymerase chain reaction (RT-PCR) revealed that TCDD elicits aryl hydrocarbon receptor (AhR)-mediated apoptosis in the avian DT40 pre-B-cell line through activation of caspases 9 and 3 (see chapter III). During the course of the HD11 microarray analyses, a consistent down-regulation of the matrix metalloprotease MMP-2 was observed. This finding was the basis for the hypothesis that TCDD has an effect on the gene expression of the MMP-2 and MMP-9 in macrophages. Then, gene expression analysis and functional zymography showed that TCDD impairs the MMP-2 and MMP-9 response to LPS stimulation in HD11 chicken macrophages (see chapter V). The microarray analyses of the embryonic bursa of Fabricius provided the basis to further study of the effect of TCDD in the chicken embryo. The shifted genes were classified according to their function. The down-regulated genes included: precursor of matrix metalloprotease-inhibitor, histone acyl-transferase 1, homeobox protein CUX-2, Death Associated Protein Kinase, and UDPglucosyl transferase, among others. The up-regulated genes included: phosphoinositidespecific phospholipase, acyl Co-A oxidase, and protein effector of Cdc42, among others. Together, these microarray analyses produced a database of genes of interest that will provide sufficient hypotheses to inspire multiple investigations aimed at confirming and refining the gene expression alterations as a consequence of TCDD exposure.