Wild-caught African Vervet monkeys are commonly infected by Simian T-lymphotropic virus I (STLV1) and Simian immunodeficiency virus (SIV), yet the natural histories of these infections are largely unknown. Seropositivity was associated with increased total, T and atypical lymphocytes. In seropositive females there was mild, normocytic, normochromic anaemia. Lymphoid hepatitis was present in seven seropositive cases. African Vervets used in biomedical research, vaccine production and organ transplantation research are often infected by exogenous retroviruses which can be oncogenic and immunosuppressive in captive monkeys. Elimination of these infections may be possible by breeding Vervets in captivity.