The effects of polymer chain ends and polydispersity in the molecular weight were incorporated into the perturbation theory based on the scaled particle theory for wormlike spherocylinder solutions, which was previously used to analyze thermodynamic properties of stiff or semiflexible polymer solutions in terms of intermolecular interactions. The modified theory was applied to three stiff or semiflexible polymer solutions, poly(n-hexyl isocyanate)-n-hexane, schizophyllan-water, and poly(gamma-benzyl L-glutamate)-dimethylformamide, to estimate the intermolecular interaction parameters (the hardcore diameter d and the strength,of the soft. attraction). The values of d estimated for all the polymers are reasonably compared with the diameter calculated from the partial specific volume. The soft attractive interaction and the polymer-chain-end effect were significant for schizophyllan, but unimportant for poly-(n-hexyl isocyanate) and poly(gamma-benzyl L-glutamate).