Concentration and electrophoretic pattern of nonhistone chromosomal proteins (NHCP) in cancer and normal endometrium were examined. In normal tissues the NHCP/DNA ratio ranged from 0.6 to 0.8 whereas in malignant tissues from 1.2 to 1.7. Mean nuclear concentration of NHCP was 14.16 pg/cell nucleus of normal endometrium and 33.35 pg/cell nucleus of cancer tissue. The two tissues differed in the electrophoretic pattern. Heavy fractions (>60 kDa — 36%) predominated in normal tissue whereas light fractions (< 30 kDa — 48%) in malignant tissue samples. The percentage of intermediate fractions (30-60 kDa) was similar. Neoplastic endometrium shows, irrespective of the patient's age, a high NHCP content. It seems that NHCP, especially the fraction extracted with 0.35 M NaCl, because of its tissue and cell specificity are involved in neoplasm formation.