A fast and simple method for the in vivo/in situ detection of liposomes is described. Utilizing lipophilic carbocyanine dyes, DiI and DiO, yellow (or red) and green fluorescent liposomes can be visualised with routinely available filters. The main advantages of the method are (i) the vesicles can be labelled after they are formed and (ii) the label does not interfere with proteins on the surface of the liposomes. Labelled liposomes were found in macrophages of spleen and liver (of mice) within 30 min of intravenous administration. In the spleen, labelled liposomes localised preferentially in the marginal zone macrophages, as confirmed by double staining with FITC-Ficoll. These data correlate well with the fact that empty or haptenated liposomes are thymus-independent antigens, and that other thymus-independent antigens are also specifically taken up by marginal zone macrophages. The immunological role of these macrophages in the processing and presentation of antigen-bearing liposomes can now be studied in more detail. Administration of high doses (1-3 mg lipid) of labelled liposomes showed that uptake occurred preferentially, but not exclusively, by marginal zone macrophages. After the marginal zone macrophages had been 'saturated', the red pulp macrophages took up the liposomes. DiI and DiO have also been successfully used for labelling lymphocytes and bacteria for in vivo homing studies. The fact that liposomes can be labelled after they have been formed is an advantage for retrospective (i.e. liposomes already in use/storage) studies in e.g. targeting of drugs by liposomes.