790 for patient 2 (the 6-year-old) are consistent with his blood-type and are distinctly abnormal, as are the absence of isohaemagglutinins in patients 3 and 4. In a series of 147 blood group 0 and A children aged 18 months and over, none had isohaemagglutinins less than 1:4, and only 6 had titres less than 1:16.’Therefore, the likelihood that the low or absent isohmmagglutinins in our patients with familial erythrophagocytic lymphohistiocytosis (F.E.L.) do not represent true deficiency in these natural antibodies seems very remote. The titres of natural antibodies to Serratia marcescens in our patients confirm these findings. In control studies, not included in the original report because of space limitations, the mean anti-S. marcescens titre of 28 children aged 2-7 years who were admitted for tonsillectomy was 1:430,2 compared with titres of 1:4,