Previous studies revealed that inhibition of mitochondrial protein synthesis by tetracyclines during a number of cell divisions results in proliferation arrest, in vitro as well as in in vivo. The tetracyclines may, therefore, be considered as cytostatics. In this context it is of interest to know the effect of the tetracyclines on rapidly dividing normal cell types, such as erythroid and lymphoid cells. To investigate the influence of the tetracyclines on the proliferation of these cells, we studied immunological responses to sheep red blood cells and recovery from severe anaemia in rats in the presence or absence of oxytetracycline (OTC). Under the experimental conditions used, effects of OTC on the immune responses were only found for events directly or indirectly related to T cell proliferation. The results presented thus strongly suggest that OTC inhibits the division of T lymphoid cells only. The proliferation of other haemopoietic cells is not influenced, most likely because these cells are not permeable to OTC.