Using HST/ACS broad- and narrow-band (Hα) images of M51 we have examined the ionizing radiation budget in a set of luminous H II regions in a spiral arm between 5 kpc and 11 kpc from the center. The extinctions within the regions were estimated, and a mean value for the differential extinction coefficient, RV, of 3.3, similar to the Galactic value, was derived. The measured AV values for the regions was in the region 0.82-3.24, with a mean value of 1.57 magnitudes. The Lyman continuum (Lyc) luminosities of the ionizing clusters were derived from their absolute magnitudes, and thence the corresponding predicted Hα luminosities from the H II regions; these exceeded the measured values by a factor of ≈ 7. Even if this discrepancy is due entirely to Lyc extinction, the range of values for this would be from 0.8 to 3.7, too small to be explained except in a highly inhomogeneous model of gas and dust distribution. Assuming that in fact 50% of the Lyc escapes from a region, the need for a high degree of inhomogeneity is even greater.