Simian immunodeficiency virus infection of macaques is a model for human immunodeficiency virus infection of humans. In vivo-titrated stocks of SIV are essential for the utilization of this model for vaccine development. The elicitation of anti-human cell antibodies by some vaccines prepared in human cells and the related protective effects of the vaccine produced in human cells suggest a need for new macaque-derived SIV stocks. Here we describe the titration and characterization of two stocks of SIV_mac that were produced in primary rhesus macaque cells. The first virus is SIV_mac251, isolated from tissues of macaque 251, and the second is a molecular clone designated as SIV_mac239. A 50% rhesus monkey infectious dose (MID₅₀) was titrated for each virus stock by intravenous inoculation. An additional five macaques were inoculated with 10 MID₅₀ of the SIV_mac251 stock and were followed for disease outcome. All five monkeys developed antigenemia by 14 days postchallenge. Two of the five monkeys developed strong anti-SIV humoral immunity, whereas three developed little or no humoral immunity. As has been observed previously, the rapidity of disease progression correlated with the lack of a strong antibody response. The three animals with low humoral immunity died within 7 months of challenge, with antigenemia, cachexia, hypoproteinemia, hypoalbuminemia, weight loss, and intractable diarrhea, while maintaining their circulating CD4 numbers. One animal died at 1.5 years of more typical simian AIDS.