Delayed toxicity of a single dose of 300 mg/kg cyclophosphamide (CP) was investigated in female DBA/2 mice. Lethality was low up to 30 days but increased markedly afterwards reaching a peak of 50% between 50-70 days with a total mortality of more than 80% by day 120 after CP. One week before death, the mice suffered a sharp loss of weight and showed typical signs of wasting disease. There was a decrease in the white cell count and lymphocyte neutrophil ratio was reversed as a result of lymphocyte depletion whereas neutrophil count remained similar to the controls. Profound lymphocyte depletion was also observed in light and electron microscopy preparations of thymus from mice with CP-induced wasting disease. Histochemical methods demonstrated increased activity of four lysosomal enzymes, acid phosphatase, beta-glucuronidase, E600 resistant esterase and n-acetyl-beta-glucosaminidase, in the thymus of treated mice. Acid phosphatase was notably active in thymus epithelial cells; the reaction product was localized in multiple primary Golgi lysosomes, Golgi cisternae, cisternae of the endoplasmic reticulum, and secondary lysosomes. The appearance of numerous cystic formations, as well as the activation of the lysosomal system and the presence of large areas of degradation support the assumption that CP-delayed toxicity is accompanied by thymus involution. Delayed mortality was partially prevented when syngenic bone marrow cells were injected as early as 24 h after CP injection. On the other hand thymus transplants were incapable of reducing delayed lethality. It is suggested that CP provokes a delayed wasting syndrome with thymic involution that is not caused by a direct effect on specific thymus structures but rather secondary to a primary injury to pre T cells in bone marrow.