The cytokinetics of the S‐180 mouse ascites tumor system was determined on Days 2, 4, 6 and 9 of growth. Studies included volume tumor growth, per cent labeled mitoses curves, and repeated injections of tritiated thymidine. It was possible to extrapolate the cytokinetic compartment values to Day 0 of growth. Results indicate that the growth fraction of the S‐180 system remains close to 1 throughout its growth, with progressive lengthening of G1, S and G2 times. Cell loss is minimal through 6 days but becomes significant thereafter. A theoretical growth curve constructed from cytokinetic values is similar to the actual volume growth curve.

The cycling characteristics of human peripheral blood lymphocytes in phytohemagglutinin-stimulated cultures were examined by means of a new technique employing radioautographic methods which assay the fraction of cells whose nuclei contain DNA polymerase utilizing exogenous and/or endogenous DNA primer-template capability. The fraction of cells labeled by these techniques increases 5-11 hr prior to the onset of DNA synthesis. Estimates of cell cycle time, G1 time, and fraction of cycling cells are made for the first 4 days in culture. Evidence is presented to support the hypothesis that daughter cells of the first divisions may be retired from cycle by virtue of loss of primer-template availability, rather than by loss of DNA polymerase.

After pulse labeling IL7~zzdln pzlr/litrrn shoot meristems \vith "H-thymidine, the cell cycle and its subdivisions \\rere determined autoradiographically using the labeled mitoses method. T h e cycle time was 20 hr. GI = 4.3 hr; C = 9.5 hr; G, = 2.9 hr; and A4 = 3.3 hr. In subsequent experiments seedlings were labeled for 4 t o 5 hr, long enough to label the chron~osomcs' full lengch, eliminating problems associated with asynchronous DhTA replication. T h e longer labeling period did not alter the cell cycle parameters.Cells were designated as being in the first or second division after labeling by the time from the midpoint of the labeling period. Seedlings were placed in 0.1% cycloheximide solution to cause sister chromatids in metaphase cells to separate from each other. Autoradiographs were then made to determine segregation of labeled D N A at the chromatid level. A t the first division after labeling all chromatids appearcd to be labeled. A t the second division after labcling \\.it11 "H-thymidine the labcling pattern was consistent \\.it11 serniconscrvative segregation of labeled D N A and with sister chromatid exchange.These results are discussed in relation to cytological and photometric studies on chromosome strandedness in L71-rrIn and in other organisms with chromosomes \\it11 diffuse ccntronlcrcs.

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