1. We report a patient homozygous for C3 deficiency and several heterozygotes from the same family. Upon follow-up, the homozygote was found to suffer several severe bacterial infections, whereas all the heterozygotes were clinically healthy. 2. C3 was undetectable in the homozygous patient, CH50 was very low and factor I was present. Serum capacity to generate chemoattractant stimuli for peripheral leucocytes was similar to that of normal adults as was also observed for one of the heterozygotes. Serum capacity to opsonize yeast was reduced in the presence of autologous and homologous (normal adult) cells. The CH50 levels of heterozygous patients were within the lower range of normality. 3. The parental consanguinity and the homozygosis state observed here are classical signs of recessive autosomal inheritance. However, the lower or below normal C3 levels detected in parents and relatives point to a co-dominant inheritance of gene S with respect to the "null" gene. 4. C3 polymorphism presented a predominantly "slow" pattern in most family members, which, together with the low C3 levels, indicates the expression of S-allotypes.