Some of the largest US bond mutual funds have invested 15% or more of their money in rarely traded securities, a practice that runs counter to long-held Securities and Exchange Commission views on the funds, an analysis by The Wall Street Journal shows.

The finding is one indicator of inconsistency in how fund managers calculate the liquidity in their portfolios, a hot-button issue for investors and regulators. By the Journal's measure, 10 of the 18 largest funds that invest meaningfully in corporate debt have significant holdings of seldom traded bonds. All the funds in the analysis said they are compliant with SEC liquidity guidelines.