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Medium calcium ions were essential to the development of the lymphocyte transformation response induced by PHA Magnesium ions could not substitute for calcium ions. Calcium exerted its influence during the induction phase of the process before DNA synthesis began since its removal after the response had fully developed did not alter subsequent levels of nucleic acid svnthesis. The early development of RNA synthesis was prevented by low calcium levels indicating that calcium of this process or some prior event(s).

Calcium uptake by normal human lymphocytes was found to be a saturable process which was competitively inhibited by manganese indicating the existence of a carrier-mediated mechanism for calcium uptake. Exchange diffusion was not observed. Phytohemagglutinin (PHA) significantly stimulated calcium uptake within minutes after treatment. The increased uptake was attributed to a decreased K,,, for the proposed membrane carrier rather than to an increased V,,,.Also PHA did not stimulate a normally unused exchange diffusion process, nor did it affect calcium efflux. Uptake by both unstimulated and PHA-treated lymphocytes was not influenced by magnesium or by cycloheximide or actinomycin D.

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