Virus concentrates of HIV-1 IIIB and Bal, respectively, were diluted 200-fold in seminal plasma and then mixed with Aquateric formulation 1 (see Methods section) at volume ratios indicated in the Table. After incubation for 5 min at 37°C, the samples were cooled in ice and centrifuged at 10,000 rpm for 10 min. The supernatant fluids were neutralized by addition of 1 M Na2HPO4. The pellets containing Aquateric with adsorbed virus were resuspended and brought to pH 7.0 by addition of 1 M Na2HPO4. Both the supernatants and the redissolved pellets were mixed with a solution of PEG 8000 (final concentration 3%). The resulting pellets were resuspended in DMEM medium and tested for infectivity as described in the Methods Section. Virus preparations diluted in seminal fluid but not treated with the Aquateric formulation represented controls.