We have analyzed 5,173 expressed sequence tags (ESTs) from three cDNA libraries of normal and regenerating intestinal tissue of the sea cucumber Holothuria glaberrima and found 22 putative immune-related genes. These sequences showed similarities (e-value approx 10(-8)) to genes involved in immune processes or expressed by immune cells. Sequences were analyzed using bioinformatic tools to determine a putative identity. In addition, phylogenetic analyses were performed to find relationships with similar proteins in other organisms. The mRNAs for ten sequences were detected in coelomocytes by using reverse transcription-polymerase chain reaction (RT-PCR). Moreover, five of them showed a significant increase in expression after an LPS challenge, while the other five showed no significant changes. These results show the variety of immune molecules that may be found in holothurians and support the idea that the invertebrate immune system is more than a collection of simple innate responses. Our study also provides new data of importance in deciphering the evolution and development of the immune system.