Trimethoprim was found to promote the uptake of thymine from Wellcotest Agar by prototrophic bacteria and by thymineless auxotrophs. However, the concentration of trimethoprim required was substantially higher in the case of the latter mutants. It was found that the minimum inhibitory concentration (MIC) of trimethoprim for thymineless mutants was very much higher than that obtained for thymine-independent bacteria providing that thymine or thymidine was present. From these results it is concluded that thymineless mutants are genuinely more resistant to trimethoprim. If levels of thymine derivatives sufficient for the growth of thymineless mutants are present in vivo then such mutants may be of therapeutic significance.