In the present series of experiments we examined the ability of eight continuous cell lines derived from human lymphocytes to release lymphokines into their culture media. We have demonstrated that all could produce a macrophage migration inhibitory factor and a neutrophil chemotactic factor. This ability was independent of the B or T cell origin of the cell lines. As a further extension of previous reports, we report that the migration inhibitory effect does not seem to be due to cytotoxic effects as judged by trypan blue exclusion studies, and more important by the demonstration that the inhibition is reversible. The demonstration that neutrophil chemotactic activity is removable by treatment with a previously described and characterized anti-lymphokine antiserum suggests that this factor may be chemically similar to some of the lymphokines (migration inhibitory factor or chemotactic factor for macrophages) produced by antigen-activated guinea pig lymphocyte cultures. The failure to effect similar removal or inactivation of the macrophage migration inhibitory factor may simply reflect greater interspecies heterogeneity in this factor, or more interestingly, that this does not correspond to "classic" MIF. These results extend previous observations by defining further the capacity of lymphoid cell lines in continuous culture to produce mediators and by characterizing their relationship to conventional lymphokines produced by antigen or mitogen-activated lymphocytes.