We report a ca se of spo n ta neous my elom on ocytic leukemi a in a n ora ngu ta n ( Pongo pygma eus). We believe thi s is th e first rec orded hematogen ou s m ali gn ancy in thi s spec ies. A Bly mp ho id cell line producin g an E BV-like herpes virus [l Z] was derived from the o ra ng u ta n.The ora ngu tan , a wild-born 13-year-old fem al e, had been at the Los Angeles Z oo since 1965. III health was first noted in F ebrua ry 1976 a nd she was kill ed 8 months la ter a fter a clin ica l co u rse characterized by slow deterioration . Sh e rece ived neither che mo the rapy , radiotherapy, nor blood transfu sion s. The hem oglobin was co ns isten tly ab out 6.0 gm /dl, plat elets were increased in number and the white blood cell co u n t ran ged between 7,000 a nd 26,000 with primitive mon onuclear cells rep eatedly found in th e peripheral blood a nd bone marrow . Cy to chem ica l stud ies on peripheral blood buffy co a t and bon e marrow indicated th e diagn osis o f myel omon ocyti c leukemia by sho wing th at ma ny o f th e a bno rm a l mon onuclear cell s stained stro ng ly for histi ocytic-m on ocytic ma rkers (ac id ph osphat ase and non spe cifi c es te rase) and for gr anulocyti c markers (Sudan black B, per oxid ase and chlo roaceta te es teras e). The persisten ce of adequate me gakaryoc yte s de sp ite de creased er ythro-a nd gran ulop oie sis in th e bone m arrow, together with the mi ld clinical signs a nd lack o f detectable orga no mega ly o r ade no pa thy , sugges ted a su ba cute form o f thi s di sease.G ross findings a t a u to psy we re unremark ab le a part from two incidental hyd at id cysts in th e liver, o ne active and o ne inactive . Spleen, liver a nd lymph nodes wer e not en la rged , unduly soft, o r di scolored nor we re leukem ic infiltrate s visible in a ny tissue . T he bon e marrow from multiple sites wa s gen erally pale and sclero tic.T issue sec tions, sta ine d with hemat oxylin and eos in aft e r fo rmalin fixati on a nd par affin embedding, sho wed unequivocal mic ro scopi c ev ide nce o f myelopro liferative di sea se. In virtu all y every o rgan, including the brain, perivascular mon onuclear leukemi c infiltrates wer e not ed ( fig . I, 2). Leukemic cell s infiltrated the bon e m arrow, lymph nodes, and splee n to a m ajor extent, de spite th e lack of o rga n en largement. In th e bone marrow the leukemic infiltrates were spotty and the intervening marrow sho wed di ffu se sligh t fibrosis a nd an ove ra ll paucity o f normal hematop oietic ce lls. In th e lymph nodes a nd sp lee n th e infiltrates occur red p rimarily in the medullary co rds and red pulp, resp ectiv ely, wi th so me spa ring o f residual lymphoid nodules. T he only infiltra tes in the live r a nd kidney were perivascular in the port al tra cts and medulla, respe ctively. C h loro ac etate esterase stai ni ng of spleen a nd lymph node sec tio ns confirmed the gr anulocytic nature o f the leu kem ic infiltrate . Extra med u llary hematopoiesis wa s not noted at any site. Sec tion s of th e inactive...