An assay was developed to quantify the growth of two different isolates of the protozoon Neospora caninum within ovine fibroblast cells in vitro by differential uptake of 3H uracil. The NC-1 isolate of N. caninum multiplied more quickly in culture than the NC Liverpool isolate, as reflected by increased incorporation of isotope by the former over a shorter period of time. After the parasites had left the ruptured host cells, there was very little incorporation of isotope. This suggested that multiplication occurred within and not outside the cells. Treatment of the cells with ovine recombinant interferon gamma for 24 h before infection significantly inhibited intracellular multiplication of the parasite.