Singlecell clones, designated E11s , C11R, and A1S, were obtained from the HuT-78 T cell line persistenty infected with an isolate of Simian immunodeficiency virus (SIV), SiVIMne. The infected clones, unlike uncloned uninfected HuT-78 ceiis, no longer expressed the CD4 marker and, after their CD3 receptors were cross-linked, had dramatically reduced intraceiiular free calcium ([Ca2+l,) responses. in one clone, E11s , the unresponsiveness was not limited to the inositoi phospholipid pathway of signaling since a reduction in CD3-mediated activation of protein tyrosine kinadependent phosphorylation also was evident in this SIV-infected clone. These resutts led us to test whether T lymphocytes from animals infected wlth SIV had defective [Ca2+], responses prior to detectable changes in CD4 levels or lymphadenopathy. The [Ca2+], responses to both CD3 mAb and CD2 mAb were 10-50% less in T ceiis from Walter Reed stage 2 animais than in healthy controls. This anergy was more pronounced in chronically infected animals progressing to Walter Reed stage 314. The responses of these animais could not be augmented even when combinations of CD3 and CD4 mAb were used. Both CD4+CD44lo T cells, which are not infected with SiV, and the CD4+CD44hl T cell subset, previously shown to be the reservoir of SIV infection in blood, had pronounced defective responses to CD3 mAb. Similarly, both CD4+ and CD8+ T ceiis were consistently unresponsive in chronically infected animals, again implying that an indirect mechanism, rather than SIV infection per se, may be responsible for this immune dysfunction. ©Oxford University Press.