Non-transformed human fibroblasts, strain PA-2, were treated with polyethylene glycol (PEG) in monolayer culture to produce multinucleate fibroblast homokaryons. Antibodies to tubulin or actin were used to monitor cytoplasmic microtubule and actin filament patterns immediately after cytoplasmic fusion, as well as after the fused cells had been in culture for varying amounts of time. The cytoplasmic microtubule complex as increased for a short time after cell fusion and then decreased to resemble the complex seen in control cells. The actin stress fibers were similarly enhanced for a comparable period of time. However, this initial enhancement of the actin stress fibers gradually diminished for approximately one month in culture after which the fibers were greatly reduced in both size and number. Concurrent with the changes in cytoplasmic microtubule and actin fiber complexes, the PEG-treated cells began to show alterations in growth parameters which progressively resembled those characteristic of transformed cell populations. Fusion of normal cells may be an initial step in the transformation of such cells to malignancy.