138 sera from renal transplant recipients were screened for the presence of monocyte-specific antibodies. Most of the sera contained antibodies against monocytes, T- and/or B-lymphocytes. One serum was identified which defined a monocyte-specific antigen, MOLI. This serum was investigated in intensive population and family studies for the estimation of the formal genetic criteria of this 'new' monocyte antigen. A gene frequency of 0.0614 was obtained by population analysis. Family investigations conveyed the information that the gene coding for MOLI was transmitted in linkage with HLA genes. A positive linkage disequilibrium of MOLI and HLA-B17 was found.