A sensitive immunoperoxidase technique for the detection of immunoglobulin (the peroxidase--anti-peroxidase or PAP procedure) has been applied to fixed smears of normal human white cells. IgM was detected in approximately 5% of lymphocytes from normal donors. Most positive cells showed a characteristic 'hairy' peripheral staining pattern; a similar morphological appearance was seen in samples stained for IgD. The membrane (rather than cytoplasmic) localization of this IgM was inferred from the redistribution of staining induced by preliminary incubation of cell suspensions with anti-mu antisera before smearing and staining. B cell-depleted and B cell-enriched suspensions showed, respectively, reduced and increased percentages of IgM-positive cells. IgG was detectable in approximately 25% of normal lymphoid cells. In contrast to the IgM and IgD reaction patterns, these cells commonly showed a discontinuous distribution of reactivity, often localized to the cell uropod or to small cytoplasmic vesicles. However, when cells were prepared at 0 degree C, staining tended to be diffuse. These findings suggested that the PAP procedure was detecting Fc receptor-bearing lymphoid cells which had bound serum IgG. IgG was also demonstrated in normal polymorphs and monocytes. The specificity of this reaction was confirmed by the use of immunoabsorbant-purified antibodies. The possible practical advantages of this immunoperoxidase procedure for the detection of leucocyte immunoglobulin are considered, and the relevance of the demonstration of IgG in non-lymphoid cells to recent reports of this immunoglobulin in Hodgkin's disease and malignant 'reticulum' cells is briefly discussed.