We present a theoretical and experimental study on the influence of a magnetic
field on the energy-level statistics in metal nanoparticles. Based on the random-matrix
theory, a gradual field-in-duced crossover behavior is predicted from the orthogonal to the
unitary ensemble. Experimental data of the electronic specific heat of metal nanoparticles
for different fields in the quantum-size temperature regime compare favourably with these
theoretical (analytical) predictions.