Inflammation is a common finding in benign prostatic hyperplasia (BPH) and may be classified as acute, chronic active or chronic inactive prostatitis. The aim of the present study was to localise the different types of inflammatory cells in prostatic lesions to determine the sequence of events in the cellular reaction. We have carried out immunohistological characterisation of the inflammatory cells, using CD45RO and CD3 antibodies to detect T-lymphocytes, CD20 antibodies to detect B-lymphocytes, CD68 to detect macrophages, kappa and lambda immunoglobulin light chains, and antibodies against prostate specific antigen (PSA) and prostate specific acid phosphatase (PSAP). Macrophages accumulated in the lumen and glandular epithelial layers of damaged prostatic glands and were found in the periglandular cuff of inflammatory cells in acute and chronic active prostatitis. Lymphocytes also accumulated in large numbers in the glandular epithelial layers and around the glands, indicating an association with macrophages. B-lymphocytes were scanty, if at all present, in acute and chronic active prostatitis, but were prominent within well-organised follicle centres in chronic active prostatitis. Cells positive for light chains were few and scattered in prostatic tissue. PSA and PSAP activity was lost in recently damaged prostatic glandular epithelium and reappeared only in regenerating secretory epithelium, indicating leakage as a result of damage. We suggest that the initial response to prostatic injury is cellular, and probably related to leakage into the periglandular tissues of PSA, PSAP and other antigenic molecules normally present in prostatic secretion. Macrophages respond, followed by recruitment of T-lymphocytes which participate in the inflammatory response and accumulate around the damaged glands. B-cell activity appears to be a late event.