Since radioiodination of human granulocyte colony-stimulating factor (G-CSF) is difficult, we synthesized a mutein of human G-CSF that retains full biological activity and receptor-binding capacity for at least 2 weeks after radioiodination. Receptors for human G-CSF were characterized in the plasma membrane fraction from the human term placenta (human placental membranes) and trophoblastic cells by using the III-labeled mutein of human G-CSF (KW-2228). The specific binding of III-labeled KW-2228 to placental membranes was pH-dependent, with maximal specific binding at pH 7.8; it increased linearly with protein to 3.7 mg of protein per ml and was both time-and temperature-dependent, with maximal binding at 4°C after a 24-hr incubation. When we examined the ability of hematopoietic growth factors to inhibit 2-5I-labeled KW-2228 binding, we found that KW-2228 and intact human G-CSF inhibited '25I-labeled KW-2228 binding, whereas erythropoietin or granulocyte-macrophage colonystimulating factor did not. Scatchard analysis revealed a single receptor type with a Bn,,., of 210 fmol/mg of protein and a Kd of 480 pM. The human G-CSF receptors on human placental membranes were shown to consist of two molecular species of 150 kDa and 120 kDa that could be specifically cross-linked to

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A bacteriophage related to R-type pyocins was isolated from a lysogenic strain of Pseudomonas aeruginosa and named PS3. It was composed of a head and a contractile tail which resembled R-type pyocins. The character of the phage was studied comparing with R-type pyocins.The activity of this phage was neutralized by anti-R or R2 serum and the antiserum against PS3 was also active in inhibiting pyocins R and R2. However, the anti-R sheath serum, while showing a strong neutralizing activity against pyocin R or R2, was almost ineffective against the phage PS3.The phage and the pyocins were further related in some way in the receptor specificity. Mutants resistant to one of the killer particles were isolated and their behavior to other agents were studied. From this result, a working hypothesis was presented with regard to the structure of the receptor for the phage and pyocins.