The 77C-->G polymorphism in exon 4 of CD45 or protein tyrosine phosphatase receptor-type C (PTPRC) has been investigated in families with multiple sclerosis (MS) and in several cohorts of sporadic patients and controls, however, with conflicting results. To better understand the role of this functionally important polymorphism in MS, we investigated the transmission of the 'G' allele from unaffected parents to their affected children in 176 families by using linkage and association statistics. The 'G' allele was transmitted to the affected offspring in five of the seven pedigrees carrying this allele and the TRANSMIT program detected association with a P=0.0342. We conclude that the 77C-->G PTPRC polymorphism is present and preferentially transmitted in a small subgroup (<5%) of MS families, which may only be detected with complementary methods of analysis.