Using British manufacturing data this paper tests Kalecki's degree of
monopoly theory of the determination of factor incomes. The paper demo
nstrates how both product market structure, captured by the rive firm
concentration ratio, and trade unionism, measured by collective bargai
ning agreements, shape production worker wage share. We report estimat
es in which it emerges that there is an important role for seller conc
entration in influencing wage share, but the estimated trade union cov
erage results are less robust. Our tests reveal that once idiosyncrati
c industry effects and accounted for the union effect on wage share di
sappears.