Angular distributions of four jets events from e + e − annihilation at LEP have been studied. Allowing variable contributions from triple-gluon vertex, double gluon-bremsstrahlung and secondary q q - production we can identify the contribution from triple-gluon vertex without comparison with a specific Abelian model. For the ratio of the colour factor N c to the ferminonic Casimir operator C F we get N C C F = 2.05 ± 0.40 −0.1 +0.7 ± 0.40 in agreement with the value 2.25 expected in QCD.