The pX region of human T lymphotropic virus type I (HTLV-I) is believed to be expressed as a consequence of a 2 step splicing. It is conceivable, however, that a donor site of the 1st splicing and an acceptor site of the 2nd splicing results in the production of an alternatively spliced mRNA which is capable of coding p21X-III. This possibility was examined by amplifying cDNA derived from HTLV-I+ cells between the 5' LTR and pX region. Bands of 2 different sizes were consistently observed. Sequencing of the longer band corresponded to a cDNA derived from a double-spliced pX mRNA as previously reported. The shorter band was derived from a single-spliced mRNA. HTLV-I+ cell lines had both mRNAs to a varying degree. Expression of p40tax and p21X-III seem to be well correlated with a double-spliced and a single-spliced mRNA, respectively.