Summary The fluorescent cell-counting technique was successfully extended to the Sendai virus and L cell system. Using this technique, the rate of adsorption of Sendai virus onto L cells was determined precisely. The hemadsorbed cell-counting technique was also devised in this system and it compared favorably with the fluorescent cell-counting technique. Because of its simplicity it is considered as a useful technique for the determination of infective virus. Evidence that a single infectious unit of this virus is sufficient to produce an infected cell, determined either by the fluorescent cell- or hemadsorbed cell-counting technique, was obtained. The fluorescent cell-counting technique was as sensitive as ordinary 50% end point titration method in eggs and its superiority was shown by its reproducibility.