In the interstitial cell infiltrates associated with primary glomerulonephritis, approximately the same cellular composition has been found, T cells being predominant, monocytes fewer, and B cells least. Cell composition did not seem to be related to the cell density; the lowest percentages of monocytes have been observed with either very high (FSGS) or very low (MGN) cell densities; and the highest percentages of T cells, with either the highest (FSGS) or the lowest (MGN) cell densities. On the other hand, some relationship seemed to exist with the cell distribution in the interstitium. The cell composition was nearly the same in diffuse and disseminated patterns, but was markedly different in nodular (focal) cell infiltrates; T cells were always in first place, but B cells instead of monocytes were in second place. Furthermore, IL2-receptor-presenting cells have been more often observed in nodular (focal) than in both diffuse and disseminated patterns, and more markedly in nonjuxtaglomerular than in juxtaglomerular noduli (foci). In conclusion, cell composition of interstitial infiltrates seems to reflect a cell-mediated immune reaction, leading to all interstitial cell infiltrates associated with primary glomerulonephritis. It is conceivable that the nodular (focal) pattern of distribution might correspond to a discrete mechanism and have a different meaning.