The monoclonal antibody (MAb) Ki-67 detects a nuclear proliferation-associated antigen which corresponds to a non-histone protein with a molecular weight of 395 and 345 kD. Its prognostic relevance has been assessed in both lymphoid and non-lymphoid tumours. The MAb PC10 picks up the proliferating cell nuclear antigen (PCNA), which is a 36 kD nuclear protein associated with the cell cycle. Whereas Ki-67 works only in fresh material, PC10 detects a fixation-resistant epitope of PCNA. Preliminary data have revealed a linear relationship between Ki-67 and PC10 reactivity in normal lymphoid tissue and in non-Hodgkin's lymphomas (NHLs). We applied Ki-67 and PC10 to frozen and routine sections, respectively, from 25 examples of Hodgkin's disease (HD) (14 nodular sclerosis, 6 lymphocyte predominance, 5 mixed cellularity) and 100 NHLs (corresponding to the main varieties of the updated Kiel classification). The results obtained can be summarized as follows: (1) both MAbs gave rise to extremely variable results within the same category of NHLs; (2) most Hodgkin and Reed-Sternberg cells (50-98 per cent) were labelled by the reagents; (3) Ki-67 and PC10 stained a similar ratio of neoplastic cells in 65 and 76 per cent of NHL and HD cases, respectively; in the remaining instances, no correspondence was observed, the PC10-positive elements usually outnumbering the Ki-67-positive ones significantly. These discrepancies, which might be due to low PCNA catabolism and/or PCNA expression by quiescent cells, underline the need for further kinetic and clinico-pathologic studies in order to define the specific relevance of PC10.