Connective tissue cells (fibroblasts) derived from skeletal muscle of 12 day old chick embryos were cultivated for almost 8 months (35 weekly passages) in rabbit plasma and rabbit embryo tissue juice diluted with Tyrode's solution. When fluids separated from these cultures were tested with immune precipitins developed against chicken serum, they gave positive reactions which showed no tendency to diminish with an increasing number of culture generations. Barring the intervention of unknown precipitable substances, these results indicate that connective tissue can produce proteins which are identical with, or closely related to, serum proteins. The experiments further demonstrated that tissues cultivated in a foreign plasma do not lose their species specificity.