We investigated the effects of carrageenans (CAR) on mouse hematopoiesis, one of the many biologic systems affected by these galactan polysaccharides. Mice were injected intravenously with potassium CAR (K+-CAR) or iota CAR (I-CAR) and studied for 7 or 14 days, respectively, thereafter. Treatment with either compound induces anemia, granulocytosis, and early profound thrombocytopenia. Treatment with I-CAR results in an early lymphocytosis, and both compounds induce lymphopenia by 18 h after treatment. Treatment with either CAR compound is associated with an early moderate reduction in the number of nucleated cells and granulocyte/macrophage colony forming cells (CFUGM) per femur. Both compounds induce splenomegaly, and I-CAR treated mice develop hypoplasia of the thymus by 18 h after treatment. The splenomegaly is associated with intense splenic hematopoiesis and an increase in the number of spleen histiocytes; many of the latter are engorged with metachromatically staining material, most likely CAR. There is a sustained increase in the numbers of spleen CFUGM after treatment with either compound; in the case of I-CAR this may be due to proliferation of CFUGM in this organ, perhaps effected by the increased levels of plasma colony stimulating activity. Although it has been suggested that I-CAR is relatively nontoxic, and, therefore, potentially useful for in vivo studies, our observations indicate that it has profound effects on hematopoiesis which must be considered when planning and interpreting in vivo studies using this compound.