Human cells (HEp-2) infected with adenovirus type 5 (Ad5) at early times (5-7 h) after infection exhibit stabilization of the filamentous actin network against disruption by latrunculin (300-2000 ng), a potent microfilament toxin. This protection is abrogated by pretreatment of infected cells with cycloheximide, suggesting that it is due to a protein induced early after Ad5 infection. Support for a role of HSP70 (heat shock protein of Mr = 70 kDa) in actin stabilization is based on several findings; (i) HSP70 is induced at early times post-infection in Ad5-infected HEp-2 cells, (ii) heat shock treatment (42 degrees C) of uninfected HEp-2 or HeLa cells results in a rearrangement of actin filaments around the nucleus, that is resistant to disruption by latrunculin, (iii) using a DNase I inhibition assay, the percentage of filamentous actin increases from 50 to 65% of total following heat shock of uninfected cells, and (iv) HSP70 induces actin polymerization from monomers in vitro.