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 : T _ { \mathrm { eff } } , \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 Å with 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 T _ { \mathrm { eff } } , \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 can not explain by itself large He overabundances in O stars .