Rat embryo fibroblasts were grown in Eagle's minimal essential medium with 10% serum and cell proteins prelabelled with L-[1-(14)C]leucine, followed by a 24h chase. When transferred to medium deprived of serum these cells showed a 2--3-fold increase in the production of trichloroacetic acid-soluble radioactivity during a 4h observation period. The microtubular poisons vinblastine, vincristine and colchicine partially inhibited this induced proteolysis, but had no effect on the proteolytic rate of cells maintained in medium with 10% serum. A similar discriminating effect on induced proteolysis was observed with cycloheximide, puromycin and insulin. The inhibitory effects of cycloheximide and vinblastine were not additive. These data support the hypothesis that, in addition to the basal turnover of cell proteins, a second mechanism of protein degradation involving cytoplasmic autophagy can be activated by nutritional step-down and is selectively inhibited by agents that interfere with microtubular function and protein synthesis.