Relationship between delayed-in-onset erythema and infiltrating leucocytes in the skin reaction site of Jones-Mote type hypersensitivity was observed in guinea-pigs. Guinea-pigs were immunized with formalin-fixed (F-SRBC) or native form (N-SRBC) of sheep red blood cells in incomplete Freund's adjuvant (IFA). Elicitation was carried out with intradermal injection of F-SRBC or N-SRBC in saline 1 or 3 weeks after immunization. One week after immunization with F-SRBC or N-SRBC in IFA, erythema accompanied by basophil infiltration was detected, but antibody production was negative. Erythema reached a peak at 24 h, but basophil infiltration reached a peak at 48 h or later. In the skin reaction site elicited with N-SRBC, high levels of basophil infiltration were detected persistently even after erythema had disappeared. Three weeks after immunization, low titres of PCA (passive cutaneous anaphylaxis) were detected in guinea-pigs immunized with F-SRBC in IFA. At that time erythematous skin reaction was detected, but the level of basophil infiltration was lower than that of 1 week after immunization. In guinea-pigs immunized with N-SRBC in IFA, skin reaction of the Arthus type accompanied by haemorrhage was observed at the site elicited with N-SRBC.