FIG. 2.

Recombinant retroviruses decay with a half-life of ∼6.5 h in fresh and conditioned media. Undiluted (solid circles) or diluted (1:100) lacZ retrovirus (open circles) was incubated at 37°C. At various times, 0.5-ml aliquots were taken and frozen. To test the activity of the virus, NIH 3T3 cells were plated at 6.0 × 10⁴ cells per well in six-well plates and the next day were transduced with 1.0 ml of lacZ retrovirus for 4.0 h at a final dilution of 1:500 in the presence of Polybrene. Two days later, the number of gene transfer events (CFU) was measured by X-Gal staining.