Under constant amplitude loading, a single variable (K) or Kmax are required in crack growth relationships. The transferability of fatigue laws, obtained under constant amplitude loading to variable amplitude fatigue, requires at least an additional variable, whose evolution with crack length accounts for the interactions effects between cycles of different types. This paper presents an analysis of fatigue crack growth tests on M(T) specimens made of a medium carbon steel. The specimens are subjected to repeated blocks of cycles made up of one or several overloads separated by a variable number of baseline cycles and two baseline stress ratios. The main objective of this study is to better understand the mechanisms at the origin of interactions effects due to the presence of overloads (or underloads) at different locations of each block loading. Results have shown that the interaction effects are closely related to the cyclic plastic behaviour of the material and also the so-called Bauschinger effect.