Positron annihilation lifetime spectra were measured for the Hep-2 cell cultures treated with the interferon (3.6×10

7

Iu/mg Pr.) and control cells. The positron lifetime spectra were resolved into three lifetime components. The clear difference of the long-lifetime components related to the pick-off annihilation of ortho positronium between the interferon affected cells and the control cells was observed. The result showed that τ

3

in the interferon treated cells was longer than that in control cells

and I

3

in the former was lower than that in the latter

but the product of τ

3

I

3

was augmented. As to free volume model

the result suggested that biological macromolecular conformation volume of the interferon affected cells was increased. The relation between such change and the anti-virus condition of the cell calls for more fuather studies.