Different effects of soluble CD4 on SIV mac239 CEMX174 vs. SIV 17E-Fred. A. For SIV 239CEMX174, sCD4 (striped), like anti-CD4 (Leu3a, gray), enhanced infectivity of stock virus, but decreased growth of late passage virus, although to a lesser degree than Leu3a. B. For SIV 17E-Fred, sCD4 appeared to have no effect on stock or P19 virus infectivity. (Note that P0 p27 values reflect day 10 culture supernatants vs. day 7 for P19).