An immunological in vitro study was carried out on conalbumin, an iron binding protein structurally similar to lactoferrin, which is a well-known bacterial inhibitor in human milk. Conalbumin itself has been proved to have bacteriostatic activity against a broad spectrum of bacteria responsible for gastrointestinal infections. The activity of conalbumin on the in vitro response to PHA, PWM and Con A and on the E-rosette formation ability of peripheral lymphocytes of 10 normal subjects was studied. The results showed that conalbumin did not affect the lymphocytes' E-rosette formation ability and did not induce blastic transformation of lymphocytes. However, conalbumin was able to produce a significant increase in the in vitro response of lymphocytes to PHA and PWM, suggesting an action on both T and B lymphocytes.