Most eukaryotic cells rapidly and specifically depress synthesis of alpha- and beta-tubulin polypeptides in response to microtubule inhibitors which cause microtubule depolymerization and presumably increase the intracellular concentration of free subunits. Other drugs which interfere with microtubule function but which lead to a decrease in the subunit pool size have little effect on the rate of new tubulin synthesis. These findings have previously been interpreted to indicate that cultured cells synthesize tubulin constitutively unless the subunit pool rises above a specified level. At this point an autoregulatory control mechanism is triggered which suppresses new tubulin synthesis through specific loss of tubulin mRNAs. That tubulin RNA levels are dramatically lowered by microtubule depolymerizing drugs is unquestionably correct; that fluctuations in the depolymerized tubulin pool size are responsible for altered RNA levels rests, however, entirely on the presumptive effects of different microtubule drugs. This caveat is not trivial, as these drugs induce gross morphological alterations, and the specificities and detailed mechanisms of action of such drugs remain poorly understood. To investigate the effect of altered levels of tubulin subunits on the rate of new tubulin synthesis in mammalian cells, we have microinjected purified tubulin subunits into cells in culture and analysed the synthesized proteins. We report here that tubulin synthesis is rapidly and specifically suppressed by injection of an amount of tubulin roughly equivalent to 25-50% of the amount initially present in the cell, thus indicating the presence of an eukaryotic, autoregulatory control mechanism which specifies tubulin content in a cultured mammalian cell line.