In this paper we present a new application of the detection of nuclear transcripts by fluorescence in situ hybridization (FISH) for studying the transcriptional activity of amplified genes in tumour cells. As a model, we have used the A431 cell line in which several amplification sites have been identified. We focused on two amplified regions: (1) the 6p12 region, which was found amplified by using comparative genomic hybridization, and which contains an amplification of the hsp90 beta gene; (2) the 7p12-p13 region, which displays a 20- to 30-fold amplification of the gene encoding the epidermal growth factor receptor (EGFr). By using FISH to detect nuclear transcripts, we show that the extra-copies of the hsp90 beta and EGFr genes are actively transcribed within the sites of amplification. This work illustrates the potential of this method as a tool for functional in situ cytogenetic analyses.