Enterovirus type 70 (EV 70) is a new type of enterovirus, which is a causative agent of "Acute Hemorrhagic Conjunctivitis" (AHC). It appeared for the first time in Ghana in 1969, and occurred in Japan in 1971. The optimal temperature for growth of EV 70 is 33-35 degrees C, and lower than that of other enteroviruses. Ninety two isolates of EV 70 were investigated to know the biological characteristics. Isolates were obtained in Japan and foreign countries in Africa, Asia and Europe in 1971-1981, and contained eighty seven from eye swabs, three from feces and two from throat swabs. Reproductive capacity of EV 70 at 38 degrees C varied from one isolate to the other. The ratios in log of 33 degrees C yield to 38 degrees C yield ranged from 0.2 to 3.2, and showed no tendency to obtain reproductive capacity at high temperature chronologically after invasion to Japan. In order to find whether EV 70 obtains the reproductive capacity at 38 degrees C, three isolates, which had high pfu ratios (33 degrees C/38 degrees C, log), 2.6-3.2, were subjected to 5 passages by plaquing method. It was found that they obtained the capacities remarkably, and the ratios decreased to -0.5-0.6. As EV 70 exhibits a physicochemical characteristic of enteroviruses, the reproductive capacity in organ cultures of human fetal small intestine was investigated. The results indicated that EV 70 isolates grew distinctly, though the degree of that was less in comparison with coxsackievirus B1 and echovirus 11, and that they increased in the capacities by passages in the intestine. These findings suggested that EV 70 developed the reproductive capacities at high temperature and that in human intestine experimentally. By neutralization tests using two kinds of antisera against EV 70, it was found that serological characteristics of EV 70 changed after invasion to Japan. Intratypic antigenic mutants were first isolated in some parts of Honshu areas in 1972, and took the place of prototype-like isolates throughout Japan in the following one or two years, and after that, no prototype-like isolates obtained anywhere. The same trend was observed in isolates in foreign countries. To investigate differences and changes of these biological characteristics of EV 70 more precisely, further observations seemed to be necessary.