for up to 7 days, The virions from the D4E1-treated culture had impaired infectivity, as measured by the 50% tissue culture infectious dose and nested PCR analysis of proviral DNA. However, these noninfectious virions were able to bind and internalize, suggesting a defect at some postentry step. After chronically infected CrFK cells were treated with D4E1 for 24 h, increased cell-associated mature p26 Gag and decreased extracellular virus-associated p26 Gag were observed by Western blot analysis, suggesting that virus assembly and/or release may be blocked by D4E1 treatment, whereas virus binding, penetration, RNA synthesis, and protein synthesis appear to be unaffected. Synthetic peptide antibiotics may be useful tools in the search for antiviral drugs having a wide therapeutic window for host cells.