Labelling with 125IUdR at radioactivity concentrations commonly employed in studied with i.v. injected tumour cells (1.0-0.1 microCi/ml) was shown to reduce considerably the in vitro reproductive viability of mastocytoma tumour cells. Velocity sedimentation cell separation studies on mastocytoma cells that had been labelled for 12 h with 0.8 microCi/ml 125IUdR yielded a population that varied markedly between fraction with respect to distribution of label and, in parallel, with respect to induced loss of reproductive viability. A similar population of mastocytoma cells that had been labelled for 36 h with 0.01 microCi/ml 125IUdR yielded fractions where distribution of label was not associated with reduced reproductive viability. Although in vivo survival (as distinct from reproductive viability) of tumour cells injected i.v. and i.p. was not significantly altered within 7 h and 30 h respectively by the commonly used concentrations of 125IUdR, it is suggested that in studies of the fate of injected tumour cells exponentially growing cells be labelled with 125IUdR for intervals well in excess of population doubling times at concentrations less than or equal to 0.025 microCi/ml.