The invention relates to assays for assessing miRNA maturation effector (preferably: inhibitor) efficacy, and to substances useful for influencing, particularly for inhibiting, maturation of miRNA. According to the invention there is provided assay of miRNA processing inhibitor efficacy, comprising the steps of: a) providing a target miRNA precursor, b) providing a potential inhibitor of one or more processing steps of the target miRNA precursor, c) bringing together of the target miRNA precursor and the potential inhibitor under miRNA maturation conditions, and d) determining inhibition efficiency. The assay of the present invention allows for a very fast and easy assessment of the efficacy of a potential inhibitor in inhibiting processing of a miRNA precursor into miRNA.