The respiratory pigments and the amino acid composition of planorbid snails which act as intermediate hosts for human schistosomes were compared with those of insusceptible snail species. The absorption spectra of haemoglobins from different snail species were similar to one another and closely related to those of mammalian haemoglobins. The occasional appearance of additional absorption bands suggested that substances other than the pigment protein appear infrequently in snail blood. Electrophoresis showed that, in general, the respiratory pigment was the only protein present in the blood. The mobility of haemoglobin and haemocyanin pigments was not affected by change in pH, and the rates of movement of haemoglobins from twelve different snail species were the same. Other proteins occurred occasionally in the blood of most species examined, and these additional fractions seemed to form a common pattern in four species. The factors determining their appearance were investigated. No qualitative differences were observed in the amino acid content of haemoglobins from different snails, but quantitative differences were demonstrated. The free amino acids of snail blood were in low concentration but were qualitatively similar in three snail species, which also contained similar bound and free amino acids in the digestive glands and ovotestes. Australorbis glabratus, however, differed from Planorbarius corneus and Lymnaea stagnalis since free methionine was found in its anterior tissues. The methionine was not present in glabratus snails infected with Schistosoma mansoni, and they contained lower concentrations of free amino acids than uninfected snails. It was shown that methionine was one of the seventeen amino acids identified in the protein of S. mansoni cercariae.