Bovine serum albumin is commonly used in many biochemical applications due to its stability, lack of interference within biochemical reactions and low production cost. Here, we model the interaction of bovine serum albumin inside a carbon nanotube. The carbon nanotube is chosen as an example to demonstrate its potential use in targeted drug and protein delivery and as an enzyme immobilizing material. We consider three possible structures as models for bovine serum albumin which are cylinder, prolate ellipsoid and three-connected spheres. Using the Lennard-Jones potential together with a continuum approach we obtain explicitly analytical expressions for the interaction energies of each configuration inside a carbon nanotube. These expressions are employed to determine the critical size of a nanotube which maximises the interaction with each model structure of the bovine serum albumin. Knowledge of the critical size is important and may be crucial for the design of a nanotube for maximum loading of the proteins and drug molecules.