A dynamometer that can be held in the hand and that was designed to measure muscle strength in a simple way in the range 2-5 of the MRC scale, was tested in order to establish to what extent differences in results can be attributed to the observer, the subject and replication. Observers learned the technique quickly and their results agreed with each other to a considerable extent. The subjects showed a small learning effect in three of the four muscles tested. Normal muscle strength was measured in one hundred 18-year-old men.