OBJECTIVE Human monoclonal antibodies have been produced against various Rh antigens. We report here the production of another one against Rh C and the first such antibody against Rh CW. METHODS Two heterohybridomas secreting IgG human monoclonal antibodies against Rh CW and Rh C (MS-353 and MS-242 respectively) were produced from alloimmunized donors. Their specificity was confirmed by serological testing. RESULTS MS-353 reacted with all CW-positive phenotypes but not with any CW-negative phenotypes, whereas MS-242 reacted with all CW-positive and C-positive phenotypes. Using 125I-labelled antibodies, the average number of CW sites/red cell was 32,000 (R1WR1W, 15,200 (R1WR1), 19,800 (R1WR2), 15,300 (R1Wr), 26,200 (r'Wr), and the average number of C sites per red cell on equivalent CW/C-positive phenotypes was similar: 22,400 (R1R1), 21,800 (R1WR1), 12,300 (R1R2), 11,700 (R1WR2), 13,400 (R1r), 15,100 (R1Wr), 21,100 (r'r), 18,700 (r'Wr). MS-353 and MS-242 were mutually inhibitory. Both antibodies specifically immunoprecipitated a 33- to 34-kD polypeptide from 125I-surface-labelled cells. CONCLUSIONS MS-353 is suitable for the serological detection of CW-positive cells, and the immunochemical data are entirely consistent with the report that the CW antigen is associated with a point mutation in the RHCE gene [Blood, 1995;86:1196-1201].