Primary gastric lymphoma, (PGL), is thought to be a tumour of follicle centre cell origin containing centrocyte-like (CCL) cells, and plasma cell components. The advent of novel leucocyte antibodies reactive in paraffin sections and improved techniques for the demonstration of immunoglobulin (Ig) in tissues has permitted a reassessment of the histogenesis of PGL. Our results have shown that PGL is a tumour of CCL cells with plasma cell differentiation in a minority of cases. Follicles were reactive, as defined by polytypic expression of Ig, in each case but selective invasion of reactive follicles by neoplastic CCL cells often led to a misleading appearance of malignancy. CCL cells bear close similarities to marginal zone cells which have been defined as a distinct non-circulating B-cell lineage. This could account for the favourable clinical behaviour of PGL.