Fragments of a pituitary tumor from a patient with acromegaly were grown in tissue culture. The tumor secreted both growth hormone and prolactin,which were recovered in high concentrations. The nonpurified hormones were characterized and compared to their respective counterparts obtained by extraction from normal pituitaries obtained at autopsy. The tissue culture and pituitary extracted hormones were eluted from Sephadex G-100 with the same partition coefficients. Growth hormone from both sources showed parallel dose-response displacement curves, by logit-log transformation, in both specific immunoassay and in a specific lymphocyte binding assay. Prolactin from both sources was compared in specific immunoassay using three different antisera. Parallel logit-log displacement curves were seen with one antiserum, while the other two antisera yielded non-parallel curves, indicating structural differences between prolactin from the two sources. Quantitative polyacrylamide gel electrophoresis was performed using multiphasic buffer systems previously developed for characterization of each hormone. By the criteria of joint 95% confidence envelopes of retardation co-efficient and relative free mobility, tissue culture growth hormone and prolactin were indistinguishable from their pituitary-extracted counterparts. This study demonstrates that, prior to purification, tissue culture derived hormone can be characterized by multiple criteria and compared to a standard preparation. Structural differences can be detected, as in the case of prolactin. When the hormones are indistinguishable, as in the case of growth hormone, it becomes worthwhile to increase the scale of tissue cultured production, with the prospect that tissue culture may serve as a source of hormone for both experimental and therapeutic use.