Volume 72, issue 3, p. 2002-2009, 1998. We reported that a single amino acid change in the V3 region of the human immunodeficiency virus envelope protein is sufficient to alter the kinetics of plasma viremia in hu-PBL-SCID mice. We have subsequently resequenced the V3 region of the 242 R5 virus stock used in these experiments and have found an additional sequence change in V3 (R to H at position 21). The virus used in these experiments thus differed from the R5X4 241 isolate (B. Chesebro, K. Wehrly, J. Nishio, and S. Perryman, J. Virol. 70:9055-9059, 1996; R. F. Speck, K. Wehrly, E. J. Platt, R. E. Atchison, I. F. Charo, D. Kabat, B. Chesebro, and M. A. Goldsmith, J. Virol. 71:7136-7139, 1997) in not one but two amino acids. We have compared the 242 H variant with the original 242 isolate (rederived from a sequenced plasmid) and have found that both use only CCR5 for virus entry and they exhibit similar kinetics of infection in hu-PBL-SCID mice. However, the 242 H variant grows more rapidly in in vitro cultures of activated peripheral blood mononuclear cells than the rederived 242 isolate. Recent experiments also suggest that 242 H and 242 show minor differences in susceptibility to CCR5 antagonists. We conclude that 242 and the 242 H variant do differ in some biological properties, but these differences do not have any substantial impact on our previously published results.