Human immunodeficiency virus type 1 (HIV-1) has been shown to involve numerous cellular factors to aid in successful viral propagation. One of these host factors is Tat-specific factor 1 (Tat-SF1). This human protein contains two RNA recognition motifs, and serves as a transcription-splicing factor. During post-transcriptional modification of HIV-1 RNA, the primary viral transcript is spliced to produce three major size classes of mRNAs: singly spliced, fully spliced, and unspliced. The relative levels of each of these viral RNA size classes are important, as they can regulate the resulting levels of viral proteins and viral infectivity. To further investigate Tat-SF1’s role as a host factor for HIV-1, the levels of different HIV-1 RNA size classes were studied in human HeLa cells with overexpressed levels of Tat-SF1. Reverse transcription and real-time PCR were then used to quantify HIV-1 RNA levels. Comparative data analysis revealed a decrease in each HIV-1 RNA size class level with Tat-SF1 overexpression, with a dramatic decrease in the levels of singly spliced mRNAs. These results support the notion that Tat-SF1 does indeed plays a role in modulating HIV-1 RNA levels in human cells.