An indirect labeling technique was used to map the surface immunoglobulin (Ig) on murine splenic B lymphocytes by freeze-etching. Cells were labeled first with fluorescein conjugates of monovalent (papain-digested) anti-Ig antibody followed by monovalent anti-fluorescein antibody coupled to ferritin. The technique avoids cross-linking and aggregation of surface Ig. Freeze-etched replicas of cells labeled at 4 degrees C, as well as of cells prefixed with paraformaldehyde showed that surface Ig was distributed in small clusters with interconnecting networks. The observed pattern was analyzed statistically by comparing it with the expected random (Poisson) distribution and shown to be non-random to a high degree of statistical significance. The deviation from randomness could be explained by the presence of clusters and relative excesses of bare membrane. Such an observed distribution of surface Ig suggests that this membrane macromolecule may be organized in a specific manner. The distribution may also play a role in the function of surface Ig as the antigen receptor on B lymphocytes.