The effects of normal rabbit serum (NRS) on two transport systems in rabbit lung macrophages have been examined. A 20 min preincubation with serum was required for the effects, which were retained for at least 40 min after serum was removed. No serum was present during the transport studies. (a) Preincubation with 0.5 or 1.0% NRS resulted in depression of lysine transport to 59 ± 2.6% (SE, 31 observations) of control levels. The activity was heat stable to 100°C for 30 min and lost after dialysis. Pretreatment with serum did not alter the intracellular concentration of lysine attained when cells were then incubated with 10 mM lysine for 30 min. The relative depression of lysine transport by serum was unaltered by preloading with such high concentrations of lysine. (b) Preincubation with 5% NRS resulted in enhancement of adenosine transport by 35 ± 2.3% (SE, 60 observations). Activity was stable to heating at 65°C for 40 min but lost at 100°C for 20 min. It was nondialyzable. Total radioactivity accumulated after 30 min incubation with 1 mM adenosine was unaffected by serum pretreatment. The two activities were separable by passage over Sephadex G25.

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