Interaction of vegetative and reproductive development was examined in Lupinus angustifolius L. cv. Unicrop, a grain legume in which only small proportions of flowers normally form fruit. Outgrowth of lateral shoots below an inflorescence coincided with shedding of flowers from upper positions on that inflorescence. Removal of laterals during early flowering of the inflorescence increased fruit number three- to fourfold, as did application to developing laterals of the growth retardants maleic hydrazide, AMO-1618, 'UNI-P-293', and 'Disugran'. Already-formed fruits appeared to restrict the number of fruit formed higher up an inflorescence, but their effect was small compared with that of lateral shoots. Shading of inflorescences due to overgrowth of surrounding laterals did not appear to affect fruit number of the inflorescence. Expanding laterals and secondary thickening of the main stem were major sinks for assimilates, while flowering inflorescences acquired only very small proportions (less than 3 %) of the shoot's current increment of dry matter. Studies of floral development showed that flower buds, open (white) flowers, and fruits were rarely shed, but that the 'corolla mauve' and 'corolla senescing' stages of late floral development were particularly vulnerable to abscission-promoting influences. This vulnerable period was marked by declining dry weight of the flower. It was not possible to determine whether the restrictive influence of lateral shoots on fruit number was of a hormonal character or due to the reduced supply of nutrients to developing flowers.