Improving biomass is an important goal for future genetic gains in yield potential in wheat, but it will also be crucial to identify traits to maximize harvest index (HI, proportion of aboveground biomass in grain). Increased grain partitioning will require enhanced fruiting efficiency (FE, grains per g spike dry matter at anthesis), whilst optimizing partitioning to increase spike dry matter growth at anthesis. There is evidence that genetic variation in grain number in wheat is associated with plant hormone levels during spike growth. For example, during booting, floret abortion is reduced by exogenous application of cytokinins, and expression of Gn1a in rice, a gene for CK oxidase/dehydrogenase (OsCKX2), was associated with grain number. Cytokinin controls cell division and lateral meristem activity, therefore CK accumulation in the spike meristem can increase grain number. Our objective was to test for associations between grain number traits and plant hormone levels of spikes in a population with contrasting partitioning traits. To do so, a high biomass association panel (HiBAP II) of 150 CIMMYT spring wheat genotypes was phenotyped for grain partitioning traits in NW Mexico under irrigated conditions. For a subset of 10 lines representative of field variation for FE in the HiBAP II, glasshouse experiments were carried out at University of Nottingham, UK in 2017 and 2018 to investigate associations between FE and plant hormone levels (cytokinins: zeatin, zeatin riboside: gibberllins: gibberellic acid (GA) 1, GA4; and abscisic acid) in the spikes at booting and anthesis. Our results showed for the subset of ten genotypes there was a significant positive association between FE measured in the field at anthesis+7d (range 38.2 - 88.8 grain g-1) and in glasshouse experiments at anthesis (range 85.7 - 124.8 grain g-1). Genotype values for plant hormone concentration interacted with growth stage for zeatin, GA1 and GA4; but genotype expression was stable for zeatin riboside and ABA. Genetic variation in zeatin levels at booting and GA1 levels at anthesis each showed a linear association with FE (P< 0.05). Selection for plantgrowth-regulator (PGR) traits will potentially allow breeders to enhance grain number and HI. However, molecular markers for these PGR traits must be established for easier deployment in breeding.