CCR5 and CXCR4 are the two major coreceptors that have been identified for human immunodeficiency virus (HIV) entry. We have modified several β-galactosidase-based HIV indicator cell lines to express CCR5 and/or CXCR4. Using these new reagents, we have been able to detect all primary isolates tested using one or both of these cell lines. However, there is large variation in the absolute viral infectivity among primary strains. Furthermore, all HIV strains are capable of causing syncytia in the indicator cells when the coreceptor is present regardless of whether they had previously been characterized as “syncytia-inducing” or “non-syncytium-inducing.”