Matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS) is increasingly used to investigate the phospholipid (PL) compositions of tissues and body fluids, often without previous separation of the total mixture into the individual PL classes. Therefore, the questions of whether all PL classes are detectable in a mixture and of whether there are significant sensitivity differences are important. Using extracts from hen egg yolk, bovine liver, and avocado, it is demonstrated that phosphatidylcholine species are primarily detectable by positive ion MALDI-TOF MS. This is due to the quaternary ammonia group and its permanent positive charge. It is also shown by using selected PL fractions obtained by thin-layer chromatography (TLC) that the sum of the spectra of the individual fractions is not equal to the spectrum of the total extract. Some PL classes are not detected in the mixture. We emphasize this result because it is relevant for PL analysis but basically holds for all compounds. (Contains 5 figures.)