The first appearance of IgM in lymphocytes varies considerably among fish species. Generally, the first appearance of B-lymphocytes and immunoglobulins is late in marine species compared to fresh water species, and larvae have reached about 20–30 mm in length when IgM is first expressed. Rainbow trout and channel catfish show the first appearances of surface IgM at about 1 week after hatching. Marine species like the sea bass, spotted wolffish and cod show IgM positive lymphocytes 1–10 weeks after hatching. Transfer of maternal antibody to eggs and embryos has been demonstrated in several species. Examples are plaice, tilapia, carp, sea bass and salmon, but not cod. The ontogeny of complement component C3 has been studied in Atlantic halibut (Hippoglossus hippoglossus L.), Atlantic cod (Gadus morhua L.) and the spotted wolffish (Anarhichas minor O.). By Western blotting experiments C3 was found in unfertilised eggs in the spotted wolffish indicating a maternal transfer. RT-PCR analysis revealed C3 mRNA transcripts from 290 d° in spotted wolffish eggs. Using immunohistochemistry and in situ hybridisation, C3 was found in liver, brain, kidney and muscle of cod larvae 2 days post-hatching and in intestines, pancreas, heart and gills at different stages of larval development. Also, C3 was detectable in halibut larvae in yolk sac, muscle, liver, brain, chondrocytes, spinal chord, eye, heart, intestines and kidney. These studies suggest that complement may play a role in generation of different organs, not only in the defence against invading pathogens. Lysozyme is a bactericidal enzyme present in mucus, lymphoid tissue and serum of most fish species, but not in cod and wolffish. The enzyme has been detected in oocytes, fertilised eggs and larval stages of fish species like coho salmon, sea bass and tilapia. The activity of other enzymes like the cathepsins has been described in eggs and larvae of sea bass, cod and salmonids. Cathepsins may have a bactericidal role in the skin of fish. Lectins are carbohydrate-binding proteins that interact with pathogenic surface structures that result in opsonization, phagocytosis or activation of complement. Lectins have been isolated from the eggs of various fish species.