Polysaccharide extracts of whole bacteria of the species Staphylococcus aureus, Bacillus pumilus, Lactobacillus plantarum and Streptococcus faecium were found to precipitate with Haemophilus influenzae type b (HITB) antiserum. This precipitation was due to the polyribitol-phosphate component of these bacteria reacting with the type b capsular polysaccharide antibodies of the HITB antiserum. Immunization of rabbits and burros with formaldehyde-fixed cross-reacting bacteria and blood-borne infections in humans caused by S. aureus (coagulase positive) induced precipitating and bactericidal HITB antibodies. Preliminary analysis of the HITB capsular polysaccharide revealed ribitol in addition to ribose providing an explanation for this cross-reaction. The ubiquitous distribution of Gram-positive bacteria containing polyribitol-phosphate in their cell-wall teichoic acid suggests that these bacteria may be responsible in part for the wide prevalence of antibodies to HITB capsular polysaccharide in the normal human population.