The present investigation addressed itself to the in situ quantification of reactive cells in tumour tissues affected by Hodgkin's disease. Immunostaining was used for identification and stereology was used for enumeration of T-helper/inducer (CD4+) T-cytotoxic/suppressor (CD8+), NK-like (Leu7+) and cells of macrophage origin (Mono 2+). The evaluation of 50 cases showed that CD4+ cells always outnumbered CD8+ cells but the degree of this predominance varied depending on the histopathological subtypes (n.s. greater than m.c. greater than l.d.). Lymph nodes contained more CD4+ as well as CD8+ cells compared to spleens. Therefore, no changes in the T4:T8 ratio occurred. No significant differences in the densities of NK-like cells were observed, comparing the different histopathological subtypes as well as lymph nodes and spleens. Similarly, macrophage (M phi) density was comparable in all histopathological subtypes. However, lymph nodes contained significantly more M phi compared to spleens. On comparison of reactive cells in Hodgkin's tissues to non-Hodgkin lymphomas (79 cases) and normal controls (7 cases) significantly higher numbers of CD4+, CD8+ and Mono 2+ cells were found in Hodgkin's compared to non-Hodgkin's lymphomas. In contrast, the density of NK-like cells in NHL as well as in normal tissues was fivefold compared to that observed in Hodgkin's tissues.