Human FS-4 cells were exposed to human fibroblast interferon for various times and further incubated in the absence of interferon until challenged with vesicular stomatitis virus. Addition of antibody to fibroblast interferon at the time of removal of interferon did not alter the development of the antiviral state. If cells were exposed to interferon for 45 min at either 0 or 37 degrees C, they developed resistance upon subsequent incubation at 37 degrees C. However, less resistance developed if the cells were initially incubated at 0 degrees C. Our results indicate that a single interaction of fibroblast interferon with susceptible cells, either at 0 or 37 degrees C, is sufficient for the subsequent development of an antiviral state, at least in the short term experiment. The kinetics of development of the antiviral state were compared with fibroblast and leukocyte interferon. The rise in the degree of antiviral resistance was steeper and maximal levels of resistance were reached sooner when FS-4 cells were incubated with increasing concentrations of fibroblast interferon than with leukocyte interferon. This suggests a greater affinity of fibroblast interferon for these cells.