We study the effect of microturbulence in the line formation calculations of H and He lines, in the parameter range typical for O and early B stars. We are specially interested in its effect on the determination of stellar parameters: Teff, log g and specially on the He abundance. We first analyze the behaviour of H and He model lines between 4 000 and 5 000 Äwith microturbulence and find that for O stars only He I lines and He Ii lambda 4686 are sensibly affected by microturbulence, and that models with lower gravities, the ones suitable for supergiants, are more sensitive to it. Using a test procedure we show that the expected changes in Teff, log g and Helium abundance due to the inclusion of microturbulence in the analysis, are small. We analyze five stars (two late, one intermediate and two early O stars) using microturbulence velocities of 0 and 15 kms-1 and confirm the result of the previous test. The parameters obtained for 15 kms-1 differ from the ones at 0 kms-1 within the limits of the standard error box of our analysis. Only later types reduce their He abundance, by 0.02 in epsilon . Comparing with values in the literature we find that the range of our changes agree with previous results. In some cases other effects can add to microturbulence, and further reduce the He abundance up to 0.04. The quality of the line fits only improves for He I lambda 4471, but not to the extent of completely solving the so-called dilution effect. Therefore our conclusion is that microturbulence is affecting the derivation of stellar parameters, but its effect is comparable to the adopted uncertainties. Thus it can reduce moderate He overabundances and solve line fit quality differences, but it cannot explain by itself large He overabundances in O stars. The INT is operated on the island of La Palma by the RGO in the Spanish Obervatorio de El Roque de los Muchachos of the Instituto de Astrof\'i sica de Canarias.