Tartrate resistant acid phosphatase (TRAP) is a reliable histochemical marker of osteoclasts when used on tissue sections of undecalcified bone. This paper presents an original morphometric analysis which can be done after histochemical identification of osteoclasts. These bone resorbing cells were demonstrated on undecalcified bone biopsies from control subjects and patients presenting a malignant disease of the lymphocyte B lineage. Computerized analysis of the osteoclastic population revealed that: (1) all TRAP positive cells along bone trabeculae belong to a osteoclastic population; (2) that B cell malignancies had an increased bone resorption. At the scanning electron microscopic level small resorption bays (about 10 microns in diameter) were observed either associated or separated from eroded surfaces presenting abnormal appearance; TRAP staining of histological sections of undecalcified bone, coupled with morphometric studies, may help in the understanding of bone disease pathobiology.