We have found that HIV-1 infection of monocytic cell lines results in a new adhesion phenotype. Whereas uninfected cells grow as single cell suspensions, HIV-infected cells grow as large aggregates. When the expression of adhesion molecules was investigated, CD44 was almost completely depleted from the surface of HIV-infected cells. Immunoprecipitation with mAb confirmed the loss of CD44 from the surface of infected cells. In addition, loss of surface CD44 was not due to formation of internal complexes or release into the culture supernatant. Soluble CD44 was not detected in culture supernatant from HIV-infected cells. Northern blot analysis showed an altered RNA pattern in HIV-infected cells. The high molecular mass CD44 RNA (7.0 kb) was lost from infected cells, and the low molecular mass CD44 RNA (1.2 kb) remained. We have previously shown that anti-CD44 mAb induces homotypic adhesion in CD44+ cell lines. In this report, we show that homotypic adhesion of the HIV-infected cells occurs through a different mechanism than anti-CD44 mAb-induced aggregation. The homotypic adhesion in infected cells was CD18-mediated, but anti-CD44 mAb-induced homotypic adhesion in uninfected cells was CD18-independent. The change in adhesion phenotype and the loss of CD44 from the surface of HIV-1-infected monocytic cells are discussed in terms of their potential implications in cell-to-cell transmission of HIV.