Surface immunoglobulin bearing (sIg+) cells were identified in the lizard Calotes versicolor by immunofluorescence using a polyvalent antiserum to lizard immunoglobulins and class-specific antibodies to lizard IgM and IgY. 53.3 +/- 1.6% of spleen cells, 23.6 +/- 0.8% of peripheral blood mononuclear cells, 21.5 +/- 1.8% of bone marrow mononuclear cells and less than 1% of thymus cells were found to bear immunoglobulin (Ig) on their surface. A similar proportion of cells in each tissue was stained with rabbit anti-lizard mu (specific for IgM) whereas only a small proportion of cells were stained with rabbit anti-lizard (specific for IgY). Adult thymectomy significantly increased the proportion of sIgM+ cells in spleen and blood whereas high dose cyclophosphamide (300mg/kg body weight) decreased the proportion of sIgM+ cells thus suggesting that sIg+ cells in the lizard are of B cell lineage as in higher vertebrates. Ontogenetic studies indicate that embryonic liver is an organ enriched for sIgM+ cells at certain stages of development and thus suggest liver to be a site for differentiation of sIg+ cells in lizard embryos.