The shaking method of harvesting human immunodeficiency virus type 1 (HIV-1) is a powerful method of obtaining high titer, highly infective virus solutions. In this method infected cells are suspended in a small volume of liquid and the mixture is shaken. Viral infectivity, measured by tissue culture infective dose (TCID50) studies, rises faster than virus titer, as measured by reverse transcriptase levels. It is postulated that this disproportionate increase in infectivity results from improved infectivity for the virus particles obtained from shaking the infected cells. Of the five strains of HIV-1 studied (IIIB, AL1212, 906, RJ4029, and MN), one strain, MN, behaved differently than the others. Upon shaking, its virus titer increased 18-fold, as opposed to the 5-10 fold increase demonstrated by the other strains. These results may indicate that MN virions are retained more on the surface of the infected cells, rather than budding off into the surrounding medium, than other HIV-1 strains. In support of this theory it was found that ratios of immunofluorescence assay scores to reverse transcriptase levels were higher for MN than for other strains.