Model spectra of protoplanetary nebulae have been calculated assuming that mass loss on the asymptotic giant branch terminates when the hydrogen envelope is almost totally depleted. The models predict spectral characteristics of protoplanetary nebulae which can be used to search for new candidates of protoplanetary nebulae in the IRAS Low Resolution Spectra Catalog. Three candidates (IRAS 10216-5916, IRAS 18095 + 2704, and IRAS 20004 + 2955) were identified in this way. Using the infrared colors of recently proposed candidates of protoplanetary nebulae as a guide, a region of the color-color diagram is suggested as the most likely area to contain protoplanetary nebulae. A search of the IRAS Point Source Catalog results in about 100 new candidates of protoplanetary nebulae. Twenty candidates are found to be associated with bright stars. Further ground-based observations are needed to confirm their nature. 45 refs.