The human immunodeficiency virus (HIV) was isolated from the blood of 192 of 410 seropositive individuals. Original isolations were made in peripheral blood mononuclear cell (PBMC) cultures, and only one-fifth of the HIV isolates could be adapted to replicate in continuous T-cell lines. Of the 192 HIV isolates, 42 had the characteristic p24 antigen marker of the acquired immunodeficiency syndrome-associated retrovirus type 2 strain of HIV (HIV ARV-2) and 150 resembled the human T-cell lymphotropic virus type III strain of HIV (HIV HTLV-III). Significantly, primary PBMC cultures from two patients yielded multiple variants. When these variants were exposed to continuous T-cell lines, only one of them continued to replicate. The remaining variants were lost and could not be reisolated following passage back into PBMC cultures. We conclude the following from these studies: PBMC cultures are more efficient at isolating HIV than continuous T-cell lines are; some patients harbor more than one genetic variant of HIV in the blood at the same time; and continuous T-cell lines are likely to yield only a portion of the HIV variants originally present in the blood.