The immunosuppressive activity of an alkylating agent Nitrogranulogen (NTG) was studied and compared with a major active metabolite of cyclophosphamide (4HCy). Both agents inhibit the human T and B cell proliferation in vitro induced by pure T cell (PHA) and B cell (SAC) mitogens. NTG is slightly less active than 4HCy and it is much less toxic to human lymphocytes. Furthermore, NTG appears to have a more selective inhibitory action against B cells.