Language processing is often analyzed in terms of two sets of factors: quantitative-distributional and grammatical (GF), with schools of thought differing in the postulated relation between the two. Our study investigates whether sensitivity to allomorphy - a GF - impacts lexical processing. Mixed effects regression models were fit to participant data from the BLP corpus with the independent variables being frequency, orthographic length, orthographic neighbourhood and inflectional entropy. A variable coding for sensitivity to allomorphy among stems was introduced and showed a significant effect on both reaction times and accuracy scores, indicating a significant role of GF in lexical processing.