The excluded-volume parameter B for brush-like polymers was calculated assuming that each brush-like segment consists of a straight main chain with Gaussian side chains. Interactions between brush-like segments were represented by the binary and ternary interactions among side-chain segments. The first-order perturbation calculation in terms of the binary interaction between side-chain segments gave a much larger value for B than the values determined from analyses of the second virial coefficient A(2) of polystyrene polymacromonomers in toluene. This suggests that consideration of higher-order terms is necessary. Results based on the smoothed-density (SD) model gave much closer values to the experimental values. Perturbation calculations considering the ternary interactions among side-chain segments revealed that it is necessary to include the residual ternary interactions for theta solvent systems. This result contradicts the experimental observation that A2 for polystyrene polymacromonomers in cyclohexane vanishes at 34.5 degrees C. However, the calculated results based on the SD model showed that the contribution from the effects of the residual ternary interactions is within the experimental error. Polymer Journal (2011) 43, 757-761; doi: 10.1038/pj.2011.61; published online 27 July 2011