Natural infection with simian immunodeficiency virus (SIV) is known to occur in the African green monkey (AGM). The actual onset of the disease has not been recognized in SIVagm infected AGM, and the precise reason for such apathogenicity in the AGM remains unclear. We reported previously that AGM peripheral CD4 lymphocytes underwent a peculiar differentiation from CD4(+) to CD4(-) cells after in vitro activation, and we inferred that the AGM does not fall into a fatal immunodeficient state because of the generation of CD4(-) helper T cells in vivo. To evaluate this possibility, we examined the relationship between CD4 expression and helper T cell activity in the naturally infected AGM. We identified a healthy monkey almost lacking CD4 T cells in the periphery. This ACM showed no signs and symptoms' of immunodeficiency and retained a helper T cell activity in antibody production comparable to those of CD4(+) AGMs. In addition, SIVagm could be isolated from CD8(+) lymphocytes in the CD4(-) AGM. These observations suggest that a unique host-virus adaptation has developed in the AGM, and may be helpful in explaining the fundamental reason for the apathogenicity occurring in this monkey.