Abstract The uptake of 3H-labelled thymidine ( 3H-TdR), 131I-lavelled 5-iodo-2′-deoxyyuridine ( 131IUdR) and 3H-labelled deoxycitidine ( 3H-CdR) by mouse spleen cells in vitro was studied using an autoradiographic method for the separate detection of 3H and 131I in cell smears. The experiment was performed in two steps. In one half of a pooled spleen cell suspension uptake of 3H-TdR and 131IUdR was compared; in the other half of the suspension uptake of 3H-CdR and 131IUdR was compared. Only two nucleated spleen cells were found which had taken up to only 3H-CdR. All other scanned cells were double labelled. Generally the nucleotides were taken up according to a rather fixed relation. It was further found that 3H-CdR and 3H-TdR were incorporated to about the same extent. However cells which were incubated with 131IUdR and 3H-TdR took up the 131IUdR to a lesser extent than cells which were incubated with 131IUdR and 3H-CdR.