Summary.-We have studied the clonogenic capacity of tumour cells in agar from 38 malignant effusions from 31 patients with epithelial tumours. Colony formation of unfractionated cells varies considerably from patient to patient, and is positively correlated with the percentage of tumour cells in the sample. Clonogenicity was shown to be reduced in 8/9 cases by removal of plastic-adherent and iron-phagocytic cells. In the ninth case, increased clonogenicity occurred after this procedure. When the autologous adherent cells were removed from the effusion and used in reconstitution experiments as an underlayer in a two-layer agar system, they were able to reverse the effect of the initial fractionation in a dose-dependent fashion. This indicates cellular communication based on release of a diffusible product of plasticadherent cells. Morphological, phagocytic and prostaglandin-synthetic analysis of the fractions involved in the reconstitution experiments implicate the macrophage as the operative cell in this interaction. However, an accessory role for lymphoid cells or tumour cells themselves cannot be excluded.

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SUMMARYRecombination of phage T4 can be stimulated more than sixfold above the spontaneous level by treatment with nitrous acid, indicating that the lesions induced by this agent are strongly recombinogenic. Two temperature sensitive mutants defective in exonuclease functions showed less than the wild-type spontaneous level of recombination even after nitrous acid treatment and a ligase mutant showed a highly elevated frequency of recombination after treatment. Since these same mutants have analogous effects on spontaneous recombination, the results imply that nitrous acid-induced lesions in DNA stimulate a recombinational repair process similar in some of its enzymic steps to spontaneous recombination.

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