Our investigations have following results: In the Rath-von Verschuer family, which is so extraordinary that the authenticity sometimes had been doubted, we found that crossing overs which were present in 50% of the cases were shown by the combination of protan defect and haemophilia B. This corresponds exactly to the observation of Whittaker and co-workers, who starting from that fact, concluded that there was a remarkable distance between the protan gene and haemophilia B gene on the chromosome. The observation enlarges the number of pedigrees with the combination of protan defect and haemophilia B. At the same time, however, we are doubtful that there might be a probable preference for this combination. With regard to the question whether manifestation of protanopia and protanomaly in any given form could be influenced by an additional factor in the X-chromosome, our present investigations give no reliable data.