A procedure for measuring chemical abundances from emission-line spectra is presented that accounts for small-scale changes in ionization, temperature, and density within the source. Applied to the spectral data for six planetary nebulae, this method appears consistent in the allowance for both filamentary structure and unobserved ionization stages. Only general indications are found for the magnitude of the correction needed to account for both collisional excitation and self-absorption effects on the He/H value. With respect to previous results, there is a decrease in the abundance of the heavier elements. The N/O enrichment reported by other investigators is found to be real and to be correlated with the population type of the objects. Applications to the study of H II regions and the gas content of other galaxies are discussed.