The influence of esterases and antiproteases on lymphocytes cultures has been investigated. Phytohaemagglutinin (PHA) and certain PHA fractions exhibited antiprotease activity. Some plant- and animal-antiproteases showed growth promoting activity in stimulated lymphocyte cultures, an effect which could be reduced considerably by incubation with trypsin. Cell homogenates and cell-free supernatants from stimulated lymphocyte cultures contained esterase activity. There is some evidence that these esterases are bound to antiproteases, a process which might be necessary for optimal lymphocyte growth.