Tubuloreticular structures that are frequently present in the cytoplasm of lymphoid cells grown in vitro were studied with cytochemical methods at high resolution. The structures are easily digested by Pronase and pepsin; they are not sensitive to trypsin or RNase. From the results obtained, we conclude that the tubuloreticular structures are of proteinaceous nature and that they are mainly composed of proteins of the acidic type. Their significance in the cell is not yet known and, therefore, further investigation is required.

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