Defective provirus genomes of human T-cell leukemia virus type I are frequently detected in lymphocytes from infected individuals and in infected cell lines. One type of defective provirus contains internal deletions spanning gag, pol, and env genes but retains portions of open reading frames for trans-regulatory proteins. The deleted proviruses could potentially contribute to viral pathology by producing novel gene products that directly affect cell metabolism or that modulate expression of resident, wild-type proviruses. Virus gene products and the control of their expression were examined in cells transfected with defined molecular clones of wild-type and defective proviruses. Internally deleted provirus clones, which are unable to produce functional Tax and Rex proteins, were transcriptionally inactive in transfected cells. Ectopic expression of p40Tax activated transcription of the deleted provirus, resulting in the accumulation of a two-exon mRNA that yields a truncated form of Rex (p21Rex). Although this two-exon mRNA also has a potential initiation codon in the tax frame, a truncated form of Tax was not detected by immunoblotting or in transactivation assays. When complemented with p40Tax and p27Rex, cells transfected with deleted proviruses accumulated an unspliced mRNA that could potentially encode gag-pX fusion proteins. Although expression of deleted proviruses was dependent on trans-acting factors produced from intact proviruses, gene products from defective proviruses did not significantly affect expression of a cotransfected, full-length provirus.