Anti-L serum prepared by immunization of a high-potassium-type (HK) (blood type MM) sheep with blood from a low-potassium-type (LK) (blood type ML) sheep contained an antibody which stimulated four- to sixfold K(+)-pump influx in LK (LL) sheep red cells. In long-termin vitro incubation experiments, LK sheep red cells sensitized with anti-L showed a net increase in K(+) after two days of incubation at 37°C, whereas HK-nonimmune (NI)-serum-treated control cells lost K(+). The antibody could be absorbed by LK (LL) sheep red cells but not by HK sheep red cells. Kinetic experiments showed that the concentration of external K(+) ([K(+)]0) required to produce halfmaximum stimulation of the pump ([Na(+)]0=0, replaced by Mg(++)) was the same (0.25 mM) in L-antiserum-treated or untreated LK cells. LK cells with different [K(+)]i (Na(+) replacement) were prepared by the p-chloromercuribenzene sulfonate (PCMBS) method. At [K(+)]0=5 mM, pump influx decreased as [K(+)]i increased from 1 to 70 mM in L-antiserum-treated LK cells, whereas LK cells treated with HK-NI-serum ceased to pump at [K(+)]i=35 mM. Exposure to anti-L serum produced an almost twofold increase in the number of pump sites of LK cells as measured by the binding of tritiated ouabain by LK sheep red cells. These findings indicate that the formation of a complex between the L-antigen and its antibody stimulates active transport in LK sheep red cells both by changing the kinetics of the pump and by increasing the number of pump sites.