DL-leucine-1-C14 was administered by intracardiac injection to guinea pigs and its in vivo incorporation into the proteins of various pancreatic cell fractions followed over a period of 2 hours.

The pancreas was homogenized in 0.88 M sucrose and fractionated by differential centrifugation to give nuclear, zymogen, mitochondrial, microsomal, postmicrosomal, and final supernatant fractions. The proteins of these fractions, obtained by precipitation with trichloroacetic acid followed by washing, were counted.

The proteins of the microsomal fraction showed the highest early specific activity and were followed by those of the zymogen and mitochondrial fractions.

The microsomal fraction was broken up into two subfractions: one consisting of detached RNP particles, the other representing mainly the microsomal content and membranes.

The incorporation of labelled leucine into the proteins of microsomal subtractions and in those of postmicrosomal fractions was studied comparatively in the pancreas of fasted and fed guinea pigs as well as in the liver and pancreas of fasted animals.

A tentative cytological picture of protein synthesis and transport based on these findings is presented.

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