A theoretical analysis is developed to compute the extensional force exerted on a fully stretched macromolecule in an elongational flow of a given extensional strain rate. The long chain molecules are described by a bead‐on‐a‐string model. The dynamic problem in the external flow field is solved by the boundary collocation truncated series method. The total stretching force is then computed from the viscous stresses on the surfaces of each bead. This approach permits calculation of the magnitude of interbead shielding effects. Current practice for estimating stretching and fracture forces on macromolecules in elongational flow experiments involves application of Stokes law to each bead as though the flow around it were independent of that around the others. In the present analysis, an analytical expression for the shielding factor is developed. An approximate formula relating shielding effects to molecular weight is extracted from the results of numerical computation. Using this formula it is proved that the free‐draining Rouse model and the non‐free‐draining Zimm model represent opposite extremes with respect to shielding effects. Also, a more accurate power index in the relationship between relaxation time and molecular weight is given. A calculation is also carried out for spectrin, the protein molecule responsible for the elasticity of the red blood cell membrane, as an application example. On the basis of a chain of 40 beads separated by 50 Å and having a radius 12.5 Å, a shielding factor of 37% was calculated. Thus, even for this relatively short polymer, the neglect of shielding would overestimate the drag on its surface by a factor of 2.7.