In our previous studies acid cysteine-proteinase inhibitor (ACPI) was shown to be a common characteristic of human squamous epithelia and dendritic reticulum cells (DRC) in lymphoid secondary follicles. In the present study, the behaviour of ACPI-immunoreactive DRC in reactive lymphoid secondary follicles and neoplastic follicles of follicular-centre cell (FCC) lymphomas was compared by the peroxidase-antiperoxidase method. The secondary follicles in reactive lymphoid tissues revealed a staining pattern characteristic of DRC comparable with the results of other studies. Contrary to this, the number of ACPI-positive DRC in the neoplastic lymphoid follicles was greatly reduced. This was occasionally accompanied by a weak reaction and in many cases the DRC were totally abolished. Moreover, morphological aberrations were observed in ACPI-positive DRC, especially diminution and shortening of dendritic processes. On occasion, ACPI-positive cell types were observed, which may represent intermediate forms between DRC and fibroblastic reticulum cells. Only one case--with a rather favourable clinical outcome (out of 41 follicular FCC lymphomas) exhibited an ACPI-positive DRC pattern comparable with that of reactive lymphoid follicles. Thus it seems that the ACPI-immunohistochemistry of DRC provides an additional tool for discerning the difference between reactive and neoplastic lymphoid follicles. This method is of particular value since it can be used with formalin-fixed, paraffin-embedded tissues. There was a tendency among the follicular FCC lymphoma patients towards a better survival rate for those with ACPI-positive DRC than for those lacking this cell type, although no statistically significant differences emerged from this limited material.