The degradation rates of 35 proteins for which primary and X-ray structural data are available have been measured following their microinjection into HeLa cells. Each protein was radiolabeled by at least two techniques, and its degradation was measured in the presence or absence of the lysosomotropic agents, ammonia and chloroquine. The intracellular location of each protein was also determined by fractionation of injected cells using differential centrifugation in sucrose or by extraction in buffers containing Triton X-100. Preliminary analysis of these data indicates the following: lysosomes play a minor, nonselective role in the degradation of most microinjected proteins; the location of an injected protein within cells may significantly influence the rate at which it is degraded; and initial analyses of protein structural data such as molecular weight, isoelectric point, alpha or beta content, hydrophobicity, etc. have not yet revealed any strong correlations with stability.