CD4+ T cells transfected with the C-terminal 130 aa of human IL-16 are rendered resistant to HIV infection. Whether the constitutively expressed IL-16 acts intracellularly, extracellularly, or both is not clear. To address this question and to further study the processing of IL-16, new constructs containing either the C-terminal 130 aa or the C-terminal 100 aa (PDZ-like motif) were constructed with and without a signal peptide. Pulse-chase experiments and treatment of cells with brefeldin A and/or tunicamycin showed that IL-16 is secreted despite the absence of a signal peptide, but with a signal peptide IL-16 is processed through the endoplasmic reticulum-golgi pathway and is glycosylated. Cells expressing IL-16 linked to a signal peptide secrete considerably more IL-16 into the supernatant than cells expressing IL-16 without a signal peptide and are considerably more resistant to HIV replication. Resistance extends to almost 25 days for cells expressing IL-16 with signal peptide as compared with only 15 days for cells without signal peptide. Cells expressing the C-terminal 100 aa not linked to a signal peptide are poor secretors of IL-16 and show little if any resistance to HIV. In contrast, cells expressing the C-terminal 100 aa linked to a signal peptide secrete IL-16 and are resistant to HIV replication. It is concluded that the secretion of IL-16 is required for HIV inhibition.