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Somatic cell hybridization techniques have allowed the preparation of interspecies hybrids that express the features of both parental cell lines. We have studied hybrids made with human myeloma cells fused to a continuous mouse myeloma cell line. In the present study we analyzed the kinetics of leucine influx and efflux in Ig producer and nonproducer hybrids. We found no statistical difference in amino acid influx; however, the rates of efflux were markedly increased in nonproducer hybrids as compared to the producers. The producer cells were tested further in puromycin known to inhibit protein synthesis. Under these conditions amino acid influx was not altered, but efflux was markedly increased resembling the findings in nonproducers. We conclude that hybrids that synthesize human immunoglobulins show decreased efflux of labeled leucine and this effect can be abolished by inhibition of protein synthesis. This difference in the efflux rate appears to be a consequence of immunoglobulin synthesis, rather than a component of a control mechanism of Ig synthesis.